I have a iPhone 3GS. Will be upgrading to the 4 when they let me (January I think).
I hit SBG once in a while on the phone, but mostly from the desktop. The interface on the iPhone is good enough so that I haven't had any issues with the site, but a mobile version with less graphics would help when not on wi-fi.
This is my 3rd smart phone since I need to do mobile email for work. Started with a Treo 600 years ago, then had an AT&T Tilt for a couple years. The iPhone is the best smartphone I've ever worked with, although the Windows 7 ones might be pretty good. Played with a prototype at an MS conference and it has potential.