Longtime Ruger P90 owner (about 19 years). Ran 24,000 round through it before I quite counting. Never a failure of any kind with any commercial ammo.
My experience with it is that it's a basic no-frills .45ACP boomstick with a surprisingly smooth DA trigger, though the SA on mine is pretty stiff. Nonetheless, the trigger has a reasonably short reset without too much creep (by comparison, the Springfield XD reset feels much, much longer). Carried it concealed for years, so much, in fact, that I wore some of the finish off of one side of the grip. Seems plenty accurate, but I don't shoot it much these days because I don't reload anymore, and .45 ammo prices are presently beyond my play budget.
The only parts I've replaced are the recoil spring (once) and the magazine latch spring (twice). Replaced the recoil spring when it became too squishy, and replaced the magazine latch spring each time it broke, about once every ten years. The only aftermarket doodads added were a pair of Hogue grips that I've long-since worn all the pebbly texture off of.
All in all not a bad gun, but not one to impress your friends with, either, unless you're a better shooter with it than they are. As a friend once described it, it's a Chevy of a gun, not a Mercedes.
My experience with other P-series pistols is limited to the P89 which, by comparison, has a heavier, not-as-smooth DA trigger pull, though the SA pull is lighter than that of the P90, though not as crisp (seems to possess a tad of squish). Nonetheless, I prefer the P89's lighter SA trigger to that of the P90's.
As others have said, decent and dependable guns capable of producing as much accuracy as the shooter. I'd carry any of them, and still do CC the P90 when the mood strikes.