I upgraded to 2 Samsung Focus', one for the wife and one for me, earlier this year using Amazon. It was amazing to me to see the $0.01 charges on my credit card. There are no catches. Just be sure you can live with the early termination penalties - which don't appear to be unreasonable compared to others. My experience was excellent. I'll never go to a store and upgrade in person ever again. It's that painless. I'd upgrade using Amazon again definitely!
Edit: Oh, I forgot to mention, my carrier is AT&T