Well, I received my iPhone about 6 hours ago. Since then, I was able to play around a bit with its GPS receiver. Unfortunately, there was only Google Maps, because no other GPS (mapping) application was available.
I'll do some closer tests tomorrow, but here are my first impressions:
It seems as it is a bit harder for the iPhone to get a fix in my living room, where my Garmin GPSmap or my bluetooth GPS receiver don't have problems. But approaching a window a bit will lead to a fix very soon...
Accuracy is displayed by a blue shaded circle that noticeable grows larger while moving away. Looks nice :)
So, first impression is "is okay, might have been better?"