I found a big disadvantage to unlocking my 8900. I purchased a new in box T-Mobile branded 8900 this past summer. This was before AT&T was selling 8900s. I unlocked it and used it for about 3 months on my AT&T account. When the display failed I called AT&T and they told me to contact T-Mobile because they could not honor the warranty. T-Mobile refused to repair it because I do not have an account with them. They suggested that I call RIM. RIM said that I had to go to the carrier to get warranty service. They do not do any device customer service.
I called T-Mobile again and insisted on talking with a supervisor. He told me that the warranty was invalidated when I unlocked the unit.
So, I bought a new lcd display and fixed it myself. Both ATT and T-Mobile, as well as RIM, washed their hands of any responsibility at all for my 8900 because it had been unlocked in order to use it on a carrier other than what it was branded.