Have to agree with Bhagwan that you can get some fabulous prices on Betfair - way above what is being offered elsewhere. Often a genuine 8/1 you can get 12/1 or better.

That said sometimes you will get significantly under if you are not careful. Lay odds can be far too long and small pools may affect placing bets - generally you have to wait until the last one or two minutes to get on with the most 'true' markets. Also you have to be prepared to 'pull' you bet if you cant get the price you are after - and place it elsewhere.

Clearly there are also bookies with Betfair accounts - as these betting exchange markets represent a very convenient way to lay off.