I've been using my Treo for a little over half a year. I find that there are a few issues that arise from time to time (random reboots every few weeks), but overall, I think it functions great. I do agree that it probably isn't just comparable to an email/voice tool. It does a lot more. I can check my email, browse the web, and reboot a server either via RDP or SSH depending on the server flavor.
I personally also like the heavy feel of it. The blackberry feels plastic and fragile. But I imagine that's a matter of taste.
As for the one handed use, that's the only way I use my Treo 650. The 5-button pad makes it very versatile. I've got a screen cover that I never open, because I never need to.
I would definitely say that the Treo isn't necessarily the best email/phone solution. But it IS a great phone/email/web/pictures/all-in-one device. And the 700 (windows mobile and palm os coming out this summer) has high speed access, as well as a 1.3 MP camera.
SO yeah, all that to say Treo's are the bomb.