This isn't "half a game" at all, it's an excellent game. The "tonne" of other stuff that other person is referring to is the DLC available with the game, just like a lot of games these days (so the person before me doesn't really understand modern day gaming).
There is already enough content on DiRT 3 without this DLC, but it was just added later on for fans to enjoy the game for longer; keeping it fresh. It's called longevity. Duh.
So ignore that person before me and buy this game if you like DiRT 2, or even if you're a fan of any old racing game, for DiRT 3 is spectacular fun.
It's not the most realistic game, but it certainly delivers a well balanced experience, between arcade and realism, with some absolutely stunning visuals. It's heavily focussed on Rally, influenced by the WRC, with a plethora of other events and game modes for you to take part in.
Multiplayer in this game is great too; you can race in any event that features in single player, so this will keep you playing right up until the launch of DiRT 4 (which hasn't been officially announced just yet).