Don't look at this as Fallout. If you look at it as Oblivion with guns, you'll have fun.
Pros:
-VATS makes combat easy, and it is so gruesome (which also means awesome)
-You'll get your money's worth
-Dark sense of humor
-Many moral chioces (do you blow up a town, or save it?)
-Not really hard, but not easy either. A nice balance.
-Interesting weapons (Alien Blaster!)
-Cool enemies (Deathclaw, Super Muties)
-Cool Allies (Fawkes, Dogmeat)
-Lots of extra missions
-Addicting
-Fun game for SP only
-Liam Neeson (Mr.Schindler, Qui-Gon-Jin [however it's spelled]) is your father!
Cons
-If you don't have DLC, it only lasts 20-40 hours (at most)
-Story is over fast, story is not that good (not bad either)
-Enemies and NPCs are sometimes stupid, + dialouge is generally horrible or annoying
-Random glitches throughout
-Unpolished graphics (some things look really good, like the water or Rivet City, some look bad...like the rocks, which are unfortunately EVERYWHERE!)
-As mentioned before, the scenery tends to repeat. Not really bad, but can get boring
-Without DLC, level cap is 20 (can be achieved in under 10 hours)
Summary: Fallout 3 is full of both good and bad, ugly and pretty. It's what you choose to see that makes it enjoyable. Rent it first, but if you like it, go and buy it (with some DLC).