Yeah...:\
I've never played the other Civs.
I know most devoted fanboiz prefer Civ4, but Civ5 is a good game is its own way.
Just don't expect it to be the same as Civ4; it's not.
I'd say it's definitely worth getting, but i don't have anything to compare it to, so it's a different perspective for me.
I suppose I will give Civ 5 another shot some day, but two attempts and about 3 hours total, it just leaves a bad taste in my mouth. While I liked the idea of city states before the game was released, in play, they are simply just a nuisance to me. I might enjoy it more if I turn them off.
The other thing, is purchasing tiles. Again, sounds like it makes sense, but it ends up being too much of a crutch. I certainly haven't progressed far enough to get the meat of it, and none of the sluggish load times when you have a large enough army.
FWIW, my favorite Civ game is probably 3, and that seems to be the one more universally reviled amongst the fanboiz. I think 4 is great, but the true beauty with it is what BTS offers. Vanilla is good enough for me, and I recognize that it is at least better than 3. It's just that I put far more time into Civ 3 than the others.