I am not a sim fan, but I am buying a wheel (it's on order) and GT5.
This is _the_ best looking game I have seen up to this point. It's absolutely insane. I have a history in both game development and feature films (9 feature films) as a CG artist and this stuff being done in real-time is pretty damn amazing. Pre-rendered still looks better of course, there are some amazing CG car videos out there that topple this, but in real time I have not seen anything better.
All other car games miss that attention to detail, touch and polish that makes GT5 look so amazing.