Sprint:
-Sprint is by far the cheapest. I get 450 minutes, N&W starts at 7pm, unlimited data, unlimited text, navigation, mobile-to-mobile, and a few other goodies for $53 a month. That's 69.99 - 25% discount which that 25% discount can be available to anybody who wants it.
-Sprint's coverage is awesome. Their EVDO speedtests are faster than Verizon's most of the time. I can't remember the last time I got a dropped call and I travel around a lot.
-Customer service has greatly improved. Oh, with their everything plans I can't really think of a reason to call customer service for billing since their rate plans are so simple now. Equipment problems with the iPhone are handled by Apple anyway.
-Any plan $69.99 or over (in other words the cheapest PDA plan) gets you enrolled in Sprint Premiere. With Premiere, you get a discount on accessories but most importantly you get a full upgrade every 12 months instead of every 24 months. Considering Apple releases a new iPhone every 12 months, with Sprint they'd sell you the new iPhones at the same prices new customers pay every year.
Sprint is the reason I own a Pre. I believe the Pre is a great phone but right now with the applications available the iPhone is the better phone but really not by a lot. I just can't justify paying for the iPhone on AT&T when I get the Pre for nearly $30 a month cheaper.