Then it would be different each person. I've blown more than one weekend on a couple of interesting Ruby issues which were semi-related to my employment.
What it comes back to though, is if you have the right value of "love to do", the pay becomes secondary to the problem. This doesn't mean that you let a company walk all over you of course, but that you are having a good time cash or no cash.
Generally one should expect to be compensated appropriately for weekend work :)
Working weekends seems like a good way to not get fully paid for doing what you love to do.