The vehicles are good, and add to the fun - especially when playing in Multiplayer mode. I like the extra game modes too.
Not sure if it's the best FPS out there... I still think that CS takes some beating for that (although haven't tried CS:S yet).
The other day I read a review in a magazine whilst walking round the supermarket (anyone else do that, or just me? It makes shopping more bearable
) that said this was so good it deserved to be a separate game and not just an add-on. Not sure if I'd go that far, but it is still a damn fine game.