I've tried other SIM's in my Rezound for voice signal strength, but not for data since the update. I want to see how other carriers work in my area and would like to just use the Rezound as the mule.
AT&T and it's prepaid "users" like Straight Talk GSM works, T-Mobile should work as well, but it takes a little trickery as the APN stuff doesn't auto-populate... In theory it should work with any compliant GSM SIM card on the stock global mode ROM, but for best results switch to a AOSP/AOKP or other open, unlocked ROM.