Sternguard: OK here's the first tricky one. Sternguard are a great answer to a lot of problems. Their special issue ammo gives them flexibility and the option to take combi-weapons on the whole squad increases this flexibility. They also can take a few heavy and special weapons, but I'm less fond of this idea as you lose the bolter and the ammo you're paying a fortune for. Which brings us to their shortcomings. They're expensive points-wise. You're paying a premium for a model with some serious killing potential, but the survivability of a Tactical marine. A small strike squad isn't a bad choice, or a drop pod with Combi-Meltas for an alpha strike can be effective, but if you want to really get the most out of these guys, bring Kantor to make them scoring. If you're investing so many points into them, at least let them park an objective.
Legion of the Damned: An expensive Tactical Marine that must deep-strike and has a 3+ invul instead of armor save. These guys got nice looking metal models, but honestly they're pretty useless. They look good on paper, but become a point sink pretty fast. If there were a way to make them scoring, I'd rate them higher, but as-is spend your points elsewhere.
Terminator Squad: The shooty variant tend to get less attention now that their assault counterparts are so effective, but these guys have their place too. They can take a Land Raider, but for these guys I'd rather footslog them and use the points to make them 10 strong. You have a big unit that's tough to kill, can throw out some hurt and then do more in CC. You can take a Cyclone Missile Launcher, Heavy Flamer (the only one in the codex not on a vehicle even though IG can take them everywhere, but that's another rant) or Assault Cannon for every 5 Termies. I like the Missile best as you get 2-4 mobile Krak or Frag missiles, plus those models can fire their storm bolters, too. Ask Space Wolves how effective Missile spam is right now and here is a great source for a few more launchers for Vanilla Marines that can take a pounding and survive.
Terminator Assaullt Squad: Here they are, in all their glory. This is one of the best units in the codex, if not the game in general. You get to pick and choose between twin lightning claws for an extra attack or the VERY awesome Thunderhammer/Stormshield. This is where things get interesting. NOw you have a mob of 2+/3+ Marines with S8 attacks. Just point at what you want to die and set them loose...there's not much that can stand up to that for very long. You lose range and they have to get into CC to be effective, but with saves like that, they can take some serious punishment and live long enough to do something. Mounting them in a Land Raider is a good option, but foot-slogging something this survivable could work...or at least draw lots of fire from an opponent that wants to kill them before they eat his lines. Again, an HQ can come in handy here...take Shrike and they get fleet...and the potential to outflank, too. These are the only cost-effective unit in the book that belongs in CC, so reach out and crush someone.
Techmarine: These guys are another example of points better spent elsewhere. If you must run a Techmarine, go with a Master of the Forge back in your HQ slots. He can do more, and can become a force-multiplier.
Dreadnought: Here's your traditional dread... Dreadnoughts have always been a good weapon in the Marine arsenal and this edition, that holds true. They gained a few weapon options and changed their basic loadout, but they still can be used a few diferent ways. First, there's the DDM (Dreadnought in a Droppod with Multimelta). This is an alpha-strike unit that can be used effectively and won't break the bank. Put it next to what has to die, and be prepared to lose the dread next turn. Adding Vulkan gives that Melta a better chance to work. Then there's what I call the support dread. These bring some heavy weapon (usually replacing the MM for something with more shots) but keep the DCCW for an effective counter-charge. Finally you have the dakka-dread. These take two long range weapons and give the army fire support. Las-ML builds used to be popular and can still work, but the dual twin-linked Autocannon is a great option here for the range, accuracy and flexibility. Take whatever your needs require and you'll rarely go wrong. I'll address venerables here too, as they're pretty much the same thing. You have the same options, but increased BS and WS, plus the Venerable rule to make them potentially more survivable. Not horrible, but only if you have the points. If I go Ven, I usually go dakka to keep him out of harm's way...he's too expensive for a suicide unit.
Ironclad Dreadnought: This is a new addition and can be a fun one too. You get a better front and side armor, and some interesting weapons. These guys work well up close so a drop pod is a good idea. Yes, you can't assault that turn, but if you place him in a good spot, you can protect him until he gets to hit something. He comes with a seismic hammer...that's a DCCW that basically counts as AP1 against vehicles. He can take a chainfist, but why? He's already strength 10. Do you really need 2d6 to roll 4? I'd rather keep the seismic hammer. Hurricane bolters are also a useless option. He can take hunter-killers, which I'd only consider to burn extra points. The Assault launchers may not be a bad choice if you're playing on a heavy terrain battle (or better in cityfight) as you get frag grenades AND defensive grenades, a rare option in many armies. If these fit your strategy, they're a solid choice.
Devestator Squad: I have issues with dev squads. They're a solid choice and a way to bring 4 heavy weapons to the field, but there are problems. First, the weapon choices are WAY overpriced. I'm a traditional guy and say stick to 4 ML guys and a Sarge. Adding extra guys as ablative wounds is an option, but I rarely do it. The other weapon choices are too expensive and cheaper elsewhere. Some people like to mix weapons, but I just don't get it. If you shoot your anti-armor at a tank, your anti-horde is useless...or you shot Heavy Bolters at a squad of Orks and then get to liquify one with a lascannon. Missiles let them deal with horde or mech effectively. They're not picking off Land Raiders, but lighter vehicles can be dealt with. My BIG problem with them is how poorly they compare to their counterparts from newer SM books. The Blood angels can bring the same 5 man 4 ML squad for 20 points less and they get Red Thirst. If that doesn't make you want to puke in your soup there's Space Wolves...They get 5 Long Fangs with 4 ML's...acute senses, counter charge and the ability to split fire...for 35 POINTS LESS! They can add another guy and a 5th Missile and they're STILL cheaper then us. So Dev squads are ok because they're what we got, but this is one place where we got screwed.
Predator: Back in the last Codex, I loved the Triple-las Pred. Now that option has gotten a bit pricey. The Dakka pred with Autocannon and Heavy Bolter Sponsons are pretty cheap and can throw out a lot of lead, but they have to remain stationary to be effective. I'm slightly less opposed to mixing weapons on this guy, but I'd say go with Autocannon/Lascannon sponson as opposed to the other way around. You can take out mech or elite infantry well, though horde won't care if you kill 3-4 models a turn. If you're looking for a cheap tank taking one without sponsons is also a decent choice.
Whirlwind: Cheap. Decent strength with mid-range AP but cover or lower strength, no cover and crap AP. These were better before. Now not so much. Moving on...
Thunderfire Cannon: This guy's interesting. He's pretty cheap and gives you several firing options, as well as multiple blasts. This is the bane of Horde armies, but can work against Mech, too. There's one shot that makes a unit roll for difficult terrain on their next movement. This is statistically a surprisingly effecient way to immobilize a Land Raider and will slow down mechanized armies. The problem comes when the Thunderfire gets shot at. It's artillary and as such potentially fragile. It can be a fun unit to field, but not the most competative.
Vindicator: Two words...Demolisher Cannon. This is a devestating weapon that can kill anything. The problem is everyone knows it, so we'll call this "Fire Magnet" Fire Magnet needs to live long enough to get in range, but once it does things start to die. Of course, one glancing hit and it's a paperweight, but it's cheap enough that running a few of them is an option.
Land Raider: Well here we are. The signature Space Marine vehicle. These guys are beastly. The basic one brings you lascannons and a Heavy Bolter, all twin linked, plus a new-and-improved Power of the Machine Spirit that lets you hit vehicles and infantry in one turn. Then the Crusader with an Assault cannon and Hurricane Bolters, plus expanded seating inside and finally the new Redeemer with AP3 template weapons on the sponsons. Now, all this is awesome until you remember my Land Raider Commandment: a Land Raider is a TRANSPORT first and gunship SECOND. Whichever Land Raider you pick, they should have something nasty inside it...usually Assault Termies. Deliver your cargo ASAP and then you can start thinking about shooting stuff. Remember, this thing is big and has a nice "Shoot me" sign on it so hiding it is a challenge and with so much melta out there AV14 isn't as unbreakable as it used to be. These can also be taken as a dedicated transport for a Termie squad, freeing up the Heavy Slot and I'd recomend doing this instead.
So there you have it. A unit-by-unit breakdown of the Great or not so great Space Marine codex. I still miss my Ultramarines, but they aren't what they used to be. I still say my issue isn't that they're not good, but that the newer Space Marine books are so much better. I haven't given up on them like I did with the poor Necrons. I'm still working out a strong list for my play style. I hope this helps shed some light on the strengths and weaknesses of this army, or at the very least I wasn't too boring.