The iPhone OS/GUI is very user friendly, as you've already figured out ;). If you want an extremely flexible device, grab a touch pro 2 if you plan on staying w/ ATT. Not sure if you will like the touch screen experience with the other smartphones, but you will have a whole host of customizations available to you with Windows Mobile that are much more restricted on the iPhone. It just isn't the same UI unfortunately.
Regarding coverage issues(?), have you actually tried your current ATT SIM card in another non-iPhone to see if it meets your needs(?) If even a dumb-phone can't get a decent signal, it sounds like you first need to find a carrier that works, then the actual handset.
Had an Apple iPhone, but have had several issues with it and coverage, drop calls, etc.
Now looking for a new smart phone to replace it and would like something that has the ability to open Office apps to view documents, etc.
I love the touch screen and web on the iPhone. Is there anything out there that can still give me that, plus the other items I'm looking for?
Thanks for your input.