I am basically in the same boat. I began a part-time masters in electrical engineering from my undergraduate institution right after I graduated with my bachelor's degree. I also do not *need* the degree for career advancement, higher salary, or anything else really, except for the satisfaction of achieving a personal goal. And work is paying for it 100%.
The reason I dove right in and went for it right away is because we wanted to start a family, but I also wanted the graduate degree, and I didn't want my daughter to only remember daddy being at school all the time. So I figured sooner was better than later. If we had already had the kid when I started (she was born between years 1 and 2 of the three-year program) that would have changed things. At this point, the only reason I am gutting it out is because I only have two classes left and I can't bring myself to quit. The time away from the family is a huge issue.
Basically, after all this rambling, it comes up to how bad you really want it. I would say you probably better want it really bad, and have a supportive family. Personally, my own motivation has been steadily declining, and my wife just rolls her eyes.