Do any of you use T-Mobile (USA) on your iPhone? What's your experience been like?

I recently inherited my neighbor's 4s (no, she's not dead, just a gift from her), and got it unlocked. I just got my sim card from t-Mobile, and was wordering if you are able to switch from a "pay as you go" option to a "monthly 4G" plan since they both don't have contracts attached to them. Also, if you're on t-mobile, what's your experience been like using the service? How are network speeds, reliability, etc.?

Actually, lets open this up a bit, if you're not using a contract on your iphone and have gone the prepaid route, how's your experience been?