Granted i'm not out on the woodsball sites very much nowadays, but if you want an opinion on anything let me know. rebellion is a great little site, friendly people and the games play out pretty well most of the time too.
as for what to expect, games are a bit more intense with higher rates of fire from people using electronic markers, and they will also stay in cover better than normal birthday party groups etc, so games will be harder.
but its good fun with games taking place throughout the day, decide how much paint you want to shoot and try and make that last, it sucks missing out on games at the end of the day. everyone gets split up in different teams on the day anyway, so just chat to those on your team and you'll fit in no problem.