I worked about 20 hours per week in a lab as an MS1 and MS2, and about 10 hours per week as an MS3 (more on lighter rotations). It paid great dividends in terms of mentorship, publications, and money. During my clinical rotations, going to work gave structure to my day, and forced me to be a lot more disciplined with regard to studying. I'm more efficient now than I would have imagined possible a few years ago.
When I started my clinical rotations, I already knew most of the people in the department I was interested in going into, so I didn't have to worry about the "first impression" effect -- I could just focus on learning. Also, since people had known me for years before I started the rotation, I got to do a whole lot more than most of my collegues. My grades didn't suffer either preclinical or clinical. All told, working really added to my medical school experience.
The downside, as you can imagine, is that when my classmates are on vacation, or enjoying a post exam weekend break, I'm usually in the lab. I don't get a good 8 hours sleep every night. Feel free to PM me if you have any specific questions.
Best,
Anka