UK O2 iPhone - No problems
The 3G works fine, I get better reception than the Nokia I use for work.
Had a few 'cannot connect your call' but I get those anyway with my Nokia N73 on Vodaphone and my Nokia on O2. Sometimes I think these complaints about the iPhone are from people who have never before experienced 3G devices.
I have also found its the most stable WiFi device I own. My Linux laptop and Windows PC take ages to hook upto wireless and quite often drop the connection, never had a problem with the iPhone, it just 'connects'.
Also on O2 you get FREE BT OpenSpace and Cloud WiFi hotspots!!
I have owned a combination of Windows and Nokia devices and the iPhone is by far the best device of them all.
All I miss is MSN chat.