I've attached a link to an article about the MIT BJ team from this month's Wired magazine. Following is a quote:

"The irony is that a bad counter often will play a more negative game than a solid player who is simply using basic strategy. One mistake per hour obliterates a counter's advantage, and two an hour is more costly than not counting at all. According to Andrew Tay, casinos know this and so rather than automatically ejecting a known counter, they'll 'watch his play, track his wins and losses, and if he's identified as a bad counter, they'll comp him a room, make him feel like a king, and laugh as his 'positive' game slowly bleeds him dry.'"

I've heard figures like this before, and they're scary. As a recreational player, I certainly can't swear on a deck of cards I don't make one mistake an hour. Are they, correct, nonetheless? That'd be discouraging, but would certainly set my sights a lot higher.