I had an account with Profit Accumulator for a year and made about £800 profit. This was with a very minimal level of use, and a couple of months where I gave up completely. I also played it relatively safe so as to avoid getting identified as placing unusual bets, and therefore banned by the betting companies (which means you can't access their free offers and therefore removes that site as being a way to make profits).
I also didn't like being anti social by sitting on the laptop while watching TV with my partner of an evening, so decided that the amount I'd be able to make in comparison to my regular job wasn't worth ruining a relationship for.
However, if you've got time to spare and/or don't mind spending your 'family time'in this way then there are decent rewards to be had, increasing with the amount of time you can spend. If I have more time on my hands in a few years then I would definitely consider starting it up again.
You don't need any knowledge of sport or gambling to get going.
The guides on profit accumulator were very good, and there are ways of negating most of the mistakes that you might make.
As you move onto casino offers then the return is not guaranteed in the same way as with the sport betting, however you then work on the principle that you'll suffer losses which will eventually be cancelled out by bigger wins off of free bets over time.