At my program (a top university program), residents who does a chief year are pretty much guaranteed a fellowship spot of any specialty at my institution's hospital. Its an unspoken "rule" of sorts, as a courtesy for their extra year of service. Most of the chiefs who done a chief year here have take this route. Chiefs who doesn't take this route usually end up at other good programs for fellowships of their choice.
Granted, not only do CR's have to not be good residents, they have to be personable, able to deal with the administration well. My gestalt feeling of how they choose (at least at my institution) is not based on your grades, your research, your inservice score, etc. It has to do with who the chairman/PD/high level administration feel like they can work with the best, and who they want as the face of the department, the program, and the institution. At my institution, CR's not only have to deal with constantly bitching residents, attendings and administration, but also have to give introduction to medical grand rounds, lead medical student in physical diagnosis rounds/lectures, and serve a lot of the department's PR function, among other things.
As far as fellowships concerned, I tend to agree that CR's tend to do well, and have a higher probability of matching in any particular fellowship prgram compared to an average resident coming out of that particular program given same credentials. But as above stated, CRs tend to be solid residents to start with (from skill/knowledge/score/pubs standpoint), and may do just as well if they dont do a CR year. But again, this may all be institution-dependent.