Like any business idea, you do need to spend time on this one and have an enthusiasm for the idea to make it work. But if you can do that, I'd say this is one of the easiest ways to make money, simply because after you put the initial few months of work in learning the systems and strategies etc, it does pretty much chug away by itself. I have some friends who make a very good income from this method. Initially they both worked on it together full time, but now she is looking after their new baby and he only works on it part time. They've managed to build a new house and seem to be able to afford nice holidays and expensive toys, so they must be doing something right!
In saying that, they did have to work on this in the evenings for a while, before taking the risk of giving up their day-to-day jobs. I know they made a few mistakes initially that had to be ironed out. It probably took them about 6 months to a year before everything finally stabilised and it became truely profitable.
Anyway - that's my two cents! :)