We've gotten three phones through Amazon and have been satisfied with the purchases. All have been on T-Mobile plans.
Looking at their
current offerings, it looks like they have phones available for as little as -$100, meaning, after all rebates are received, you'll have received rebates totaling $100 more than what you paid.
The best deal I ever got was -$70, plus a free Jabra Bluetooth headset when purchasing a SonyEricsson T610.
I also noticed that you can get a free 3-Pack accessory kit with the purchase of some phones right now, in addition to all the rebates.