I think Forza is great, I played the demo of Dirt and thought was was ace as well. It all depends on if you love racing real cars and tweaking them in a realistic way. I can see why Forza gets a bit dull after a while, but I'm still playing it. The time trials and exhibition stuff is fun, and adds a bit. Online is good as well.
In my view, Forza 2, Dirt and PGR 3 are all as good as each other, just quite different in style. Pick which type you like.