Medical,
When I was in high school I did an internship in a neuroscience lab because I thought I wanted to go into medical research. I really really disliked it, but then later realized it was because I sat on my ass for 8 hours a day since there wasn't much I was qualified to do - I was signed on to handle computerized data analysis, but the implants never worked the whole time I was there.
So I gave it another shot in college, mainly because I needed the money so I did a summer internship/program in the Dept. of Physiology and BioPhysics during the summer of my sophomore year. That turned out to be really interesting - my project involved localizing particular calcium channel subunits involved in epilepsy. At first I was terrified - I was afraid I'd mess things up, calculate dilutions and stuff wrong, whatever. But it ended up being a really good experience. It does help in your classes...especially if you're a biology major. Everyone and their mother does Western blots (or Northerns and Southerns) and so when the time comes that you learn about those techniques in say...your molecular cell bio class, you'll understand it and have first hand knowledge with how it works. It'll also keep fresh the basic chemistry (mainly stoichiometry or chemical equilibria or pH stuff) that you'll need for the MCAT. It also helped a bit with anatomy and things that have appeared on the MCAT before (what a neuron looks like, basic anatomy of brain regions, etc.)
That summer, the data I collected turned out to be very good and it's their best to date, so for 10 weeks of work...I'm getting published although not as first author or anything.
After that summer I went on a year abroad and came back this year to the same lab. Now a whole other host of opportunities have come up - a project with NASA in which particular antibodies might be taken up to space with the astronauts (not quite sure what this is about), and a project with the Navy to generate antibodies to Anthrax. Right now I'm not sure exactly what I'm doing because I just got back to school, but have been doing some derivitization of antibodies (fancy name for the process of coupling antibodies directly using fluorophores) and I'm not sure where that's going to lead yet. But this year it has helped refresh certain basic science concepts that you tend to forget over 4 years, plus I see applications in the chemical bio course that I'm taking (although physiologists take a different approach...many people studying physiology are biochemists).
Anyway, in a long answer to your question...it's helpful and if you choose a lab that you think researches something fascinating, it can be a very rewarding experience. They just liked me because I listened to music and danced while working, so I was a "fun wild child" compared to the absent-minded professor types.
