I did a part-time master's in CS while working full time. I was able to take one course a term and it was really rewarding. I could dig into the material for each course without worrying about 2 or 3 other classes - really nice. I also wrote a thesis rather than just taking credits. It took me a lot longer (1.5 years start to finish on thesis), but it was immensely rewarding. I learned so much beyond my thesis + class topics - a lot more than I expected when I started out. Also helped that my employer paid for the whole thing!

Never thought a part-time PhD was possible (US), but maybe its worth thinking about.