I moved from a Treo 700 on Verizon to an Iphone with ATT. So far, I have not been disappointed.
ATT's coverage has been good for me, and I travel a lot.
I was on Verizon's hi-speed network with my Treo and while the Edge network is a bit slow, it's not that much slower than my Treo was. Wireless is flawless. It connects transparently to any open network and tells me about locked networks it finds.
Email works fine. There is no mass delete...yet, so that is sort of a PITA, but I just deal with my spam folder when I'm at a computer. I use IMAP and have not had a single problem. Calendar and email syncs perfectly with my Outlook 2007 client.
Call volume is fine for me (and after years in a rock band, my hearing isn't what it used to be) and the cool part is that when you're using the earbuds, you hear the caller in both ears so it sounds even better...more like a live conversation. I don't use the earbuds for listening to music though...I either use my Sennheisers or my V-MODA buds.
I don't use it much as a camera so flash is not important to me nor is the ability to take videos.
I also don't do a lot of text messaging. It does have an SMS client that I haven't even used yet. I'm old school...if you want to talk to me, call me ;-)
The keyboard took a bit of getting used to, but I'm actually faster on this now than I was my Treo.
My battery life is more then acceptable. I use it a lot and charge it about every 3-4 days.
The cool factor is amazing. I'm still blown away by the interface.
So, for me, it's the best phone I've ever had...
-- Bill