I have Airvoice's unlimited plan for $30...only get a notification the last couple days before it expires...which I appreciate very much.
What kind of data rate are you getting? What metro area?
Apparently the AV annoying notification was for the METERED (per minute) plan.
I ended up with Red Pocket because web people says better data rate, I notice some improvement on my 4s, not major, but also RP has many international destination FREE, included in the metered rate which really works for me.