www.easysportbet.co.uk have a new betting exchange product named Betkeen, the commission is lower than betfair (average 3%) and all odds are the same as betfair.
+ every week they give a bonus if you lost money. I think this is the best chance to be a winner in the long term.
Personnally last year I had a good year with a total profit of +1200 euros, but of course some weeks you will lose, so a bonus when you lose is really a big plus. I highly recommend Easysportbet.