I am enrolled in a co-op program right now... at my school, there is a required co-op program where students rotate every three months from going to class to our co-op jobs for the duration of your undergrad. I'm majoring in MechE, but love the biomedical sciences, and my first three co-op terms were doing biomedical research at a top medical school. I actually quit that job (crazy for a pre-med, I know, it was animal research, and I disagreed with their ethics + lack of safety protocol) and now work at a National Laboratory, doing research on hydrogen storage materials.
I think it will help my application quite a bit. My second job is slightly less relevant research (more ChemE stuff), but by the time I earn my degree, I'll have 2.5 years of full time work in separate fields of science research. I know that my experiences have been more intense than the undergrad summer students because my boss can give me long-term responsibilities, plus a 10-week summer working on a study that will last for 10+ years really limits how much of the process you can see. The longer you work, you're trusted with more responsibilities, your chances of being published go up immensely, your co-op supervisor can write a much more tailored rec letter, you earn money two or three times that of a summer job, you aren't distracted by school stressors/imminent tests, etc. I regret going to my school in some ways (my school doesn't even have the major I want, and really limited adv. Bio class options), but it's given me this co-op edge that is extremely unique, particularly for a pre-med student. Also, I thought I wanted to do PhD school when I first started working, now I know it's not for me long-term!
It will end up working out well for me - if you can get a job in a research field, or a biomedical industry one, I think it will look great.
