I love cockpit in most games, but in GT5 it frustrates me because it feels tacked on and not well done. For instance, a lot of the cars' rearviews are impossible to see out of, either because they're covered by HUD stuff or pointed in the wrong direction or off screen. The driver's perspective feels far away and too low so seeing the corners while racing in a tight pack gives me a headache.
I think the view in the 908 cockpit is beautiful. Unfortunately, the mirror is facing sideways and there is no LCD video screen like there is in the real one. So I usually (begrudgingly) switch back to the front-grill cam or whatever it is, because it gives you a nice big rearview mirror and is a lot safer in traffic.
I also think fast lap times are a little easier without the cockpit in the way, even though that could be considered borderline cheating.