I can't speak from experience but when it comes to research fellowships, you need to figure out if it will help your application and/or future career path.
If you have low board scores, poor clinical grades, and no research experience to date, a dedicated year off for research isn't likely to make a huge difference for your application. On the other hand, if you have an average to above average academic record, but are lacking in research, a dedicated year off could absolutely help you stand out as an applicant and increase your chances of matching at your preferred programs.
For some students, another motivation for a research year is that you are interested in a career in academics, and want to jump start that process by spending a year in research. This is hit or miss in my opinion: if you get a great mentor, and work in a productive lab, you can really help yourself; but if it's a suboptimal situation, you might end up regretting it.
Lastly, I think it's important to mention that if you think your chances of matching are so-so, and you are considering a research year, you should plan to take the research year between MS3 and MS4. This allows you to sell the narrative to programs that you were planning ahead, and were interested in a research experience, instead of settling into research in the event you don't match.
