Based on the single player mode review, i preffer the DiRT2 better.
That said, the games is solid, the graphics are stunning amazing, cars looks much better IMO. All in all, i miss the adventuring racing rally feeling that the D2 have, no interaction with other racers like it have in D2, which makes the races kinda boring and repetitive. The game is way more challeging than D2 thought.
Tracks are okish, so far not much diversification. The night racing is fantastic also are the weather like snow and rain.
Another things that bothers me in D3 is that the settings like Dificult selections are presets, most of the time you'll need to customize what you want to balance it on your racing skill level. Making it Easy will make it more like a touring for the tracking than racing, it automatically breaks for you on curves, the point is, even if you pick a super fast car, you wont be able to kick the meters to extreme since it automatically adjust the breaking/accel/etc...
I played it mostly on Casual, but even on casual it made my racing so boring that i had to customize the settings, turnning off the track "path" that leads you throught the track, for example.
Another thing, i miss the damn old trailer i had as my home in the DiRT2... now its some fancy shiny triangle crystal thing that floating on the screen, that is really boring. I know that it doesnt affect the game but hey, it felt more like a rally game.
Resuming, if you are thinking in buy this game just for casual game play in single mode, i suggest you to stick with D2, way more fun, interacting with other races and such.
That's it, back to DiRT2.