There are many things i'd like to say about GT5, and it's a great game! But maybe a bit too "great"? 
To quote Jeremy Clarkson on some car he talked about. "It's like Scarlet Johanson, it's so pretty that you wouldn't know what to do with it, you'd rather just sit back and admire it". 
 
And i think thats the problem, its so pretty and well made, its Jay Leno's wet dream, but in terms of pure fun, it never quite reaches where Forza 3 took its franchise with Community services such as marketplace where you can sell cars, setups etc.  
This also made Forza more approchable for the masses i think since you could just buy a good setup, a cool paint job of a lolcat and be the most casual racer ever, in a quite passionate racing game. 
 
I think the decals in Forza added loads of personality aswell. 
 
So as of now i am really enjoying GT5, but its got a job to do. It should get some tattoo's and piercings before having a knife fight with Forza, and the GT5 that emerges will be fantastic =P