I too am a former iPhone/Att user who converted when the Hero came out.
While I appreciate the iPhone for it's innovation and "just works" factor (really should say "just works as long as you do what we say" but that is another discussion) I really could no longer justify ATT's outrageously high prices. Not to mention their complete lack of 3g coverage.
As far as Customer service goes I think Att wins but only barly and this is mainly due to the fact that you get transfered to an iphone person for technical help when you call. While Sprint does have a few "bad apples" left in Customer Service, in general all you do when you get someone who is being unreasonable is just call back, really it works 9/10 times.
For me the main reasons to support/stick with Sprint over anyone else are:
1. Price (only T-Mobile is close in this category and there rural coverage BLOWS)
2. Openly touting 4G as truely "unlimited" this at a time when Att is cutting your kneecaps at 2gb of data.
3. 3g is at 5GB still with no additional "tiering" in sight (hello Verizon we know your going to screw us just like Att)
4. Price (did I mention that already?)
5. They "roam" on Verizon. Even though your roaming data is slow and limited, voice is not. So if you need to make a call or send an e-mail you can do it almost anywhere in the country and at almost 1/2 the cost of what Verizon will charge you. Good luck doing that on ATT or T-Mobile.
All that being said enjoy Att I'm sure your won't have any trouble...