I've tried Sprint, AT&T, Verizon, Cingular, what else?!? Hospital use is extremely limited function by order, (depends on where you work), but it's mainly only as a "page" function while on grounds, and where I work now does not allow iPhones, period, (though no one really knows why).
Doesn't matter for me anyway, however, because I'm a tight-wad, and I would never pay that kind of money for a phone, no matter what it does. AT&T charges are also absolutely absurd, and I make a great deal of international calls.
The T-Mobile plan I have now is by far superior to all the other carriers I've used, and the customer service is also the best I've had, (love those T-Mobile folks!). The phone is not comparable in functions to other "smart" phones, but perfect for my needs, and popular where I am in the medical communitites especially.
It's small and thin, and while the texting is a bit more precise with the size, (comes with a plectrum, but I haven't needed it), it's perfect for the pocket in a lab coat or scrubs. I also go through them quickly, (lose them often, get abused at work), and the price doesn't hurt. It's a Nokia Nuron ... no big whoop to the tekkies out there, but just what is needed for me, and reliable.
T-Moblie rocks.