Haven't time to reply in detail, but the issue is frequency bands. You need to make sure they match your network, and different US networks use different bands. They'll work for 2G, but how well for 3G may depend on network and location. If you compare specs for unlocked, ATT & T-Mobile versions you'll see the diffs.
Sprint are different in that they use CDMA rather than GSM, so it's a completely different standard and compatible at all. If you are on Sprint, you need the Sprint version.