They're a pre-paid company who rides on the T-mobile system.
If you have a T-mobile-compatible phone, ya might get 3G speeds (or even 4G speeds).
If you have an iPhone, which doesn't do all the Tmo frequencies, then the best you'll get is 2G (edge) speeds.
Last time I checked, they had a reasonably unlimited deal for about $40/month.
You'd have to verify.
For a while, I was considering using my iPhone on 'em, and even bought a sim on Amazon.
Think it was $2, including shipping.
($1.79; ordered the 5th, received the 7th, first class letter)
That's your start-up cost... that and a month worth of service.
I am currently evaluating Straight Talk.
Higher start-up costs ($50 + $45 first month), but carried on AT&T.
My iPhone gets real 3G there, without any problems.
There is extensive discussions of these and similar carriers on another forum.
I hope we'll eventually bring some of that knowledge back here and post it.