The "doughnut" effect is very distinctive, and some folks don't care for it. Also, the mirror lenses have a fixed aperture (typically f8 or f11) so you will be limited in your shutter speeds, if you're using the lens handheld with slower film.

I got pretty inexpensively the newer 500/8 Reflex-Nikkor, and I like it a lot. It focuses very closely, which gives it a great deal of versatility, and is quite lightweight, so may be a nice fit with the FG.