I used one for a couple of years, not that much as 90mm is not my favorite focal length, but the results were always (i.e., unless I messed up) sharp and contrasty. It is SOOOO small. I ended up selling it and buying a 90mm Elmarit because I found that for portraits I wanted a bit less DOF.
However, I regret these buying and sellings - in retrospect, I should have kept the 90mm Rokkor for lightweight/travel, and bought a 90mm Summicron pre-ASPH for portraits. On a positive note, the 90mm Elmarit is a small compromise on both fronts, but alleviates the problem of which lens to carry. It's a good all-arounder.
In summary, a great lens, as are the other 2 M-Rokkors!