A great cross of simulation and arcade racing style makes it an incredibly fun game with lots of variety to offer!
Dirt 3 offers a variety of game modes, ranging from circuit racing to rallycross with some technical turns and obstacles to overcome, as well as the Gymkhana mode.
The races only last about 2 minutes each, which keeps the pace of the game up and the variety flowing as well. The issue I found with most racing games is the 5-6 minute races just get boring after 2 or 3 minutes; that's no issue here.
There are a variety of amazing looking environments to race in from snow to dirt, and all of them have well laid out tracks for the types of cars racing, and the type of race.
The physics engine was also very well done, having a cross between simulation racing (that is too much realism for me to have fun), and standard racing games (that has not enough realism to me).
Some great online lag free modes (Such as outbreak and cat and mouse- youtube them), in which you can host custom games, makes the multiplayer fun and able to keep replay value; the only demerit is that it can sometimes be hard to find a match that has more than 3 or 4 players.
Dirt 3 is an all around great game with lots to offer, with no mentionable flaws; It runs smoothly and offers a great mix of race types and has a perfect physics engine to top it off.