I think a lot of your ability to work depends on the load you're taking. I, for one, spent pretty much ALL my spare time in the computer lab my first three years of undergrad. I was certainly not a partier--maybe the occassional movie or sporting event for sanity's sake, but that's it. Finally, when my schedule calmed down a bit I took on a part-time 20hr/week job my senior year (still taking a full load of classes). I worked full-time during the summers to help compensate for the lack of working during the school year.
I know working a full school schedule and full work schedule can be done, but I don't see the overall benefit of doing that if you don't have to (so long as you already have a good work ethic). Frankly, I felt burnt out by the time I was done and didn't fully recover sleep- and heatlh-wise until months after it was over. More power to you if you can do that.