I switched from AT&T to Straight Talk, then to Airvoice because the $10 plan was cheaper for my low usage. Now, my usage is not as low, and I added a Skype subscription to reduce my Airvoice usage. I was still using about $20 per month with Airvoice, and juggling making calls over Skype, leaving data turned off (missing group texts and MMS messages and making it a hassle to turn data on when no wifi is available), and topping up my Airvoice balance outside the normal automatic monthly renewal.
Back to Straight Talk for me, I can dump the Skype plan, leave my data turned on, not worry about doing manual refills in the middle of the month, and never come close to using the 5 GB data allotment.