There is a great advancement in the communication devices today including the Apple's Unlocked iPhone 4. It seems to bring more comfort to the user because it's going to deal with less hassle compared to other phones which are married to a specific carrier. In terms of network carrier, I am more convinced with AT&T. This company has given excellent services to a lot of clients and I don't give a doubt on them. One advantage you may acquire from using the unlocked iPhone is when you travel internationally because you may use a micro-SIM card that you can purchase from the local carrier.
Link: Apple's Unlocked iPhone 4: A Getting-Started Guide | PC