FWIW: It seems that undergrad research is the most underappreciated activity on my app. Only at Vandy did they ask about it, and that's because they're hard up about research.
Anyways, here's what I did (BTW: I am going for MD only, and do NOT intend on becoming an academic...I did research b/c I enjoyed it as an undergrad, but I got my fill):
-Freshman year - 1 credit research assistant (aka lab b*tch). Here's where I learned a lot of neat stuff about the organism my lab was devoted to studying. - 3-5 hrs/wk. No papers or anything.
-Soph summer - served as a Summer Fellow (stipend and research grant money) for 8 weeks at my college. I designed my own experiments with an advisor and carried them out. 40 hrs/wk. Paper, but not published.
-Senior year - departmental honors. I was responsible for managing a lab, submitting a budget / orders, training lab b*tches
and, oh yeah, getting bench time myself to run all the experiments I designed. It was an awesome experience b/c at my college they treated bio honors kids like mini-PhDs. Consequently, it was 20-40 hrs / wk for the first three quarters...and, well....a lot more near the end. My paper was 12.5k words and I had to give a committee defense (which sucked). In addition, another honors student and myself presented our research at the ASM meeting this year...so that poster took a long time to prepare. All things considered, it was WAY more stress than I needed when I should have been winding down. It would have been impossible to pull this together if I was taking 16 credits (I took 12 and audited another 4 in anticipation of the workload). The most fulfilling things were designing experiments and training future honors kids. From talking to other liberal arts college friends of mine, my experience is not unique. They had similarly tough/rewarding honors experiences. I cannot speak to many large universities, but the friends of mine who did attend those types of school didn't have the same type of "mini PhD" experience.
Bottom line: if you're at a small school, or at a large school that lets you be independant, definately do research. I could walk into any lab in med school and own....solely b/c of my undergrad research experiences. All that being said, I don't want to do much, if any, research in med school
...bench work just isn't my deal.
If you are just going to be somebody lab monkey, then maybe it's not worth it. I guess in that case doing a semester or a year might help out and give you some technical skills and maybe a letter (and probably from a big name if you're at a big school). You should investigate cases like this for yourself.
Best of luck deciding!