I heard about Straight Talk this morning from Clark Howard (guest on KFI 640AM). It's $45/month for unlimited Talk, Text & Data (bring your own phone). They use all the major network carrier's networks (Verizon, AT&T and T-Mobile). So as long as your phone works on one of those networks, it will work through them and you should see no difference. They don't offer new phones. It's strictly "bring your own."
In some cases, even if you're in a contract, it might be cheaper to pay an early termination fee and switch.
So say you just got a new phone 3 months ago and you're paying $100/month. With Straight Talk, you'd save $55/month. A Galaxy S3's Early Termination Fee on AT&T is about $300. In 6 months, you'd save $330 - $300 etf or $30. In 21 months, you'd save $1155 - $300 etf = $855. All while getting unlimited talk, text and data.