I have a 929 and a 986 but I don't shoot any competition. They both are excellent range and plinking guns. I like "gadgety" stuff sometimes and a 9mm revolver is all that. They are really best if you reload. You can shoot light 9mm loads that won't cycle an auto, load without much concern for OAL, and shoot most any 9mm bullet. A 9mm revolver is handy for using up ammo that doesn't do well in your autos. You might be surprised at how snappy lots of factory ammo is.
The main caveat in 9mm revolvers is adequate bullet tension. 9mm ammo can have bullet creep in a revolver. A heavier gun like a 929 or 986 is usually ok but bullet creep is a real problem in light weight revolvers, even with factory ammo.
I think the main advantage of a 929 or 986 is fast reloads in competition. They really don't do much that a good 38/357 revolver won't also do. The fun factor, though, is pretty high!