I did the full time ATPL groundschool at BCFT.
I entered it with the intention of working part time, mainly at the weekends, but found I just didn't have the time.
It really depends what you want to get out of the ATPLs. If all you want is 14 passes at 75%, then you could probably hold down a part time job aswell. I wanted to know the subjects well, so studied as hard as I could. I found i spent most nights during the week studying, and took one day at the weekend off. Finished with a 93% average, so I'd say it payed off.
The main problem is the volume of information you have to take on board, and the sometimes bizarre wording of the CAA questions.