I rented Motorstorm and man is it ever fun with all the available vehicles. I will definitely be buying this one. It isn't exactly 100% realistic, but it IS very, very fun. The sound kicks butt on this game too.
I tried the RR7 demo and thought it sucked - yes fun, but so unrealistic that it wasn't something that I would continue to play, and even the graphics weren't that impressive after seeing GT-HD and Motorstorm. I deleted that one after a few plays. Same with Full Auto demo - fun, but not something that would keep me entertained for long.