Affordability: A-. There's a lot of great deals out there, it's just the large number of metal models holding them from a higher rating.
Tactics: Mixed, but specialized. Unline a Marine Tac squad, you are not the jack of all trades. You can build your list for close or long range killing, but if a unit is cought outside its designed role, bad things tend to happen.
Useless Unit:Heavy weapons teams. They have some cool guns, but they're artilliary, which sucks in 40K and they take up Heavy slots that are better used for other units. Honorable mention here is the Wraithguard. They're a great unit, but another one that's prohibitively expensive. $100+ for a single unit is just stupid.
Final Grade: B+. They're good, and easily collected. They look good in almost any paint scheme. The problem is they are a tricky army to play well, but as you get more experienced you'll learn how to pull off combos needed to win.