When the game first came out I enjoyed it, played the hell out of it, But then Forza 4 came out.
Forza 4 is the better game by a long margin, The cars actually sound like the real thing, Making designs for cars, I believe it has better modifications and i'll probably get some stick from some of you hardcore ps3 fans (even though i own a ps3) Visualwise forza looks soooo much better.
What I like from GT5 is the better choice of Tracks to race on, Better choice of cars (even if they're in crappy standard form) (Wish they made the Honda Accord Euro R a Premium) and the best thing about GT5 is the photo mode, Photo mode turns everything near enough real life, took a picture of a DC5 showed it to loads of people and they really believe its a real life picture, its awesome.