Get it unlocked from O2. I had no problem getting my contact 3GS unlocked free of charge (took about a week and was done over the air), but if its a pay as you go they'll charge you and you'll lose the unlimited data tariff. Not a problem if you're going to move to vodafone though.

Battery life is woeful. You can extend it a bit by turning the backlight down and turning off 3G and Bluetooth if not required.