in the long run, it is a losing deal because you pay more than 10 cents for each half point (usually 20 cents once you buy more than 1 point), and each half point is worth less than 8 cents (normally). Playoffs, however, have been a little closer to the line than long term average and regular season, so you would have been lucky so far. But luck doesn't last forever...
My advice: get multiple books, including ones that give you a free half point, take positions and catch line moves, and you will have positive value expectation middles....
