I have a friend that uses them. He seems generally happy with them except for one thing...their "visual voicemail" seems flakey as hell. He will often not get notices of new messages until a couple hours after they are left and sometimes not at all. I am just talking about the notifications. If he manually checks the app and tells it to refresh, the messages will usually appear.
At first glance it seems like a phone problem (it is a phone he got through Metro PCS) but they have been unable to come up with a solution.
I did a search on the problem and saw others were also talking about it. It doesn't seem limited to a particular phone.
-KeithP