How good is Betfair? If you bet only European soccer it's good, but not perfect. If you bet only American sports, it's good to have it one of your outs, but not the only one.
For average bettors Betfair's commission rates stays always over 4%. You have to be a high roller to get 2.0-2.5% commission.
If you get odd 2.00 (+100) from Betfair, it's actually 1.95 to 1.98 (-105 to -102), so sometimes it's better to lay your bets with Pinnacle or some other low vig bookie than betting exchanges.