There is a good review here http://www.pokerlistings.com/betfair-poker

I played there for a while and their cash games are OK but I didn't like the tournaments, as the blind structure was designed for speed and I felt that didn't suit my game.

Before signing up, it is worth looking in to rake back. This is one site that I have used and they have a deal for Betfair http://www.rakeback.co.uk/betfair.htm