I just did the exact opposite. I went from AT&T to Sprint, got an EVO, my first Android phone. Android on AT&T is an absolute joke, this is the main reason I switched. I just didn't like any of their phones. They are so madly in love with the iPhone that I believe they are purposely limiting Android phones on their network, they want no competition to Apple in house.
When I dug a little further and compared prices between the 4 major carriers I realized Sprint had MUCH better value.
Also, I live in a large metropolitan area, and I have noticed severely poor network performance from AT&T at large gatherings like sporting events. I'm talking 90 minutes for a text to get out. Forget about calls. This may be a minor issue to most people, but I work many of these events, so it's a big deal to me. My coworkers on other carriers, have dealt with slowdowns, but all were surprised with how bad mine was.
My advice, if you can deal with Apple, get an iPhone, but be prepared for network slowdowns in large cities. If you really want an Android phone, run, don't walk to Sprint or Verizon.