Yes, I gave it a go a few years ago and made about £400 if memory serves me correctly. I found that once I ran out of sign up offers on the more well known sites (I wasn't keen on signing up to the dodgy looking ones), it got a lot trickier, and by last couple of months i would have been better getting a minimum wage job for the amount of profit I made vs the hours I put in. Maybe I was doing something wrong.
The only real risk is human error either in betting on the wrong thing or not fully understanding how a particular bet works.
Also, I wouldn't look at it as a long term thing as eventually the betting sites catch on to what you are doing and your account gets banned.
Hope this helped.