I switched to Ting in August.
The "catch" is that you pay retail for the phone. (not subsidized by two year agreements like on most carriers.) You can stop Ting anytime with no penalty. Even "suspend" your account if you don't need it for a while.
So I paid $240 for A Galaxy S2.
With 2 days left till my bill, I have used 101 minutes, sent 287 txts and used 60Mb of data -- so unless I suddenly make 400 minutes of calls, send 700 texts or download 40Mb of data, my bill will be $23.
(Yes, I'm a light user -- and I tend to do most of my surfing/downloading when I've got wi-fi)
I've calculated that when I switch over my wife and daughter too, I will be saving $100+ per month over AT&T. Even with paying full-price on the phones, it'll pay for itself in about 6 months.
The bad thing is that Sprint is not a great choice outside of major metros. Up here in New England, I'm roaming pretty frequently -- no cost for voice on that, but Ting doesn't "roam" data, so access to wi-fi becomes more necessary.
Overall, for $100/month savings, I'm willing to put up with a little aggravation when it comes to signal coverage.