Using the 4636 hidden menu, with stock GB, people have been able to get GSM working - but I have read the Rezound's radio might not support all GSM frequencies.
People have been successful getting voice calls working - both receiving and originating calls.
Texting is a problem, and DATA seems to need jumping through many hoops and may not be reliable.
You can read about it in this rather long thread:
[Q] Global Phone [CONFIRMED] (SOLUTION MAY BE ON THE CARDS SOON FOR SMS) UPDATE 12 - xda-developers
The general feeling is that the Rezound's hardware makes it a Global phone, and that Verizon's software interferes with this. The Pokemon ROM is working to get these Global Phone capabilities working better, but the last I have read it is not completed.
It would be nice to have the capability to change the SIM card and have a working cell phone while visiting another country. It is also possible that some people may want to save money and after their Verizon contract is expired move their Rezound phone to another service like T-Mobile.