I have been with Sprint for 9 years through the sucky coverage with good customer service and vice versa. They now have all of their ducks in a row from my perspective. I get great coverage and the customer service has improved immensely.
I signed up for the 450 minutes with unlimited texting about a month ago with AT&T to give the iphone a whirl. I was using an ipod touch. My signal in D.C. is nowhere near as good as Sprint. I have decided that I like having constant internet for the apps though. However, the iphone stinks as a PDA. Furthermore, for $10/month with Sprint, I get unlimited everything.
I'm going to give the Pre a whirl for the PDA aspects and the service. I'm going to put my iphone on a data only plan. If the Pre gradually gains most of my favorite apps. I will go back to the ipod touch full time.