I did the HHMI fellowship (the one where you can do at any school). I really thought it was a great experience. Of course it depends on the lab and mentor you will choose, but I really, really enjoyed my year. I ended up having 2 first author papers, one second author, another 4th author paper and a few abstracts. I did think about doing post-sophmore fellowship but I was really more interested in doing research for that year. I did get my first choice for residency, but I am not entirely sure I needed the HHMI or psf for that 🙂 although it probably helped. Feel free to ask me more questions.
Also, your research does not have to be very basic science oriented. Mine was more of a translational research and was more clinically oriented.