I'm doing one now. What's great about these faculty development fellowships is that you can continue working, being the awesome attending you are, and still do a fellowship on the side to learn to become even more awesome. I feel like it is very useful. It can open up a lot of opportunities in academics to publish, be a leader, get research, etc.
In terms of competitiveness, I do not think it's competitive at all. Not all are interested in doing it, and some fellowships out there are really "in-house" fellowships that only applies to current faculty in the medical school. I have friends who have done fellowship outside of their academic institution, where they would travel to another institution for a week or so throughout the course of 1-2 years, but they all had full backing by the department they are currently working at. Basically if you really want to do one, you can get it easily, as long as your chairman approves of it! (This is all assuming you are already working at an academic institution).