If it's something that interests you and you'll get legitimate research experience (ie. not washing beakers) then do it. As tigress said, you could potentially get a great LOR discussing your work ethic, ability to think critically, interact with others, etc. As tigress also pointed out, you will stand out and have something to talk about in interviews (this could be positive or negative, most likely fairly neutral) as long as you're prepared to discuss it.
The other benefits, as I see it, are:
You learn research methodology, stats, critical thinking. Maybe you get a pub which is never a bad thing.
You get exposed to research early so that you have an idea going into medical school if it's something that interests you. Many people need to decide whether to spend their M1 summer doing research or clinical activities; if you've already decided where you stand on research, you've made that decision (just make sure that it's research you do/don't like and not the particular research you do in UG).
Bottom line--do it. LOR, experience, $$$. Do you need more reasons? Adcoms don't.