Ultrafly is the shoe Topo builds when you need support, and it is the one Topo itself files as a stability shoe rather than neutral. The support comes out of the shape of the platform rather than a hard post, so it guides your foot instead of fighting it the way old motion-control shoes did.
It suits you if you overpronate, if your form falls apart at the end of a long run, or if a neutral shoe leaves your arches sore the next morning.
It is not fast. At 281 g on a 30 mm platform it is built to hold you steady across a lot of easy miles, not to change pace. It has also been superseded, so buy it for the ride rather than for being current.