It seems to yield pretty good results from what I understand, and I know a lot of teams in a lot of race series are using it to improve gear toughness in R&P and transmission gears.
Some of the places will re-heat treat first to bring the hardness down before cryo treating. This can supposedly get some pretty good gear toughness while reducing brittleness. It's difficult to say what the results will be, because if you're not wearing out parts or breaking stuff then you won't know if there's any improvement with less wear, less breakage, etc. It's really a matter of how good/complete the original heat treat was, as far as how much the cryo treatment helps to complete that heat treat process. If the heat treat was very good, there may not be a lot of residual austentite to convert to martensite (that's the core of the process really) so you may not get much of a change.
Brake rotors are a different story. They have a lot of pearlite because they're cast iron, so the effects are different from regular steel where you're turning residual austentite into martensite.
Like anything, do your research about particular shops and the processes that they use.
It's a pretty cheap process, so if you're interested and curious then you should give it a shot. Also look into REM finishing and shot peening. Playing with materials is fun, go for it....
