It's 4 bucks, but might be worth the cost if you're having signal issues with Sprint and want to force your phone onto another network. I think there's a way to also do it by manually updating your PRL#, but I don't know how to do it. I saw a guide somewhere, probably on XDA.
I used to use roam only for the Evo 4g and loved it but I dont want to root my Samsung Epic 4g touch as it is brand new and would prefer to leave it stock. It will only work if you root the phone.
I can vouch for Roam Control. Worked like a charm on my Evo. Have not used it on the GS2 though as it requires root, and I'm not ready for that adventure yet.