i'm doing my PhD full time and teaching two undergraduate classes, and i've got a little job at our local careers service. the teaching is fun, it usually takes me about a day to prepare, plus I have 6 contact hours/week. i am definitely paid less hours than i work, but in a sense I don't mind, as i am also learning, and do enjoy it.
the little job at the careers service is great. it's two hours/week but i can do more if i want, and it's really well paid, and though it's intense while it lasts, it's really limited to those two hours; no preparation or such. nice extra money without too much limitation to my PhD.
overall, i feel it's manageable - though as has been said before, if you can avoid having to work, then don't.