I have the 650 on SprintPCS.
It does most of what I wanted - apps, email/web, games, music (with a large SD card).
It failed on IPSec VPN - they removed support a few weeks before I got it, but didn't bother mentioning it. Most people wouldn't care about this - but securely pulling up my work servers on my phone would be useful (customer support calls and such)
How do you feel about your phone rebooting? It does not do it during a call, but every so often (maybe once a week) switching apps it'll reboot (takes about 5 secs.)
The small amount of onboard memory makes upgrades a pain - in my case I backup, purge the unit, upgrade, restore. It should not be that hard.
I will probably pass this one on to my son when my contract is up and get something else (if it's "cool" enough).
I hear better things about the 700....
It really depends what you want it for, as a basic everything device it's ok. But for me it's like guns...I always want more....