I used to for about 6 years until about 2 years ago on Ultimate Bet, Bodog, and a little at Full Tilt. Mostly small stakes No-limit up to $25 NL. Did a small amount of bonus whoring-moving money from site to site to take advantage of different site's deposit bonuses. Not a ton, but I added a little to my bankroll doing that. There are websites devoted to bonus whoring.
It was only a hobby. A hobby that made me a little bit of money though.
When it quit being fun as a hobby, I pulled out all of my money. It just got to be too much like work.
I got out before the you know what really hit the fan when the Justice Department shut down some of them, at least temporarily. I haven't really checked back to see if they are up and running again. Not much desire to really get back into it.
I know for a lot of people it was more than a hobby. Judging by what people posted on some of the poker forums, some people were really good at it and made a decent living. I know I never had an inkling to sit in front of a screen playing multiple tables all day long. If the poker gods started crapping on you, things could get ugly.
Enjoy though! It is fun and as they say, Texas Hold-em takes a minute to learn and a lifetime to master.
