I've been weighing this option myself, and I cannot thank the users of this forum enough for the invaluable information. Without it, I definitely would stick with my iPhone 4.
Although CREDO has limited phones, they are offering $200 towards buying out your contract with your current carrier. AT&T iPhone users have a $325 ET fee, less $10 per month for each month of the contract used. Plus, iPhone 4s are selling for $500 locked and $600+ unlocked on eBay. So whereas one could go with another carrier and still pocket some cash, it's more advantageous to go with a company like CREDO who will help with the ET fee.
I don't have a Hero (yet), but judging from everything I've read, the iPhone is the superior device in many ways, yet limited by Apple and the iTunes store. I would venture a guess that an unrooted Hero is lightyears behind any iPhone, but once rooted and updated to 2.2, it could have enough added features that the iPhone lacks to make it a viable replacement.
Again, for me it's about getting away from AT&T and making a sound financial decision. Granted, I'd love to have a newer Android device. With 22 months left on two 2-year contract, it's much more appetizing to get the $400 from CREDO and live with the inferior phone. I'm sure they'll add new devices in the future.
My question is, even though CREDO says you cannot use other handsets with their service, would it be possible to unroot said device and flash it with a different ROM to make it usable with CREDO?