Not a past owner, but I got to shoot one quite a bit.
First off, it was a reliable gun. Never had a jam or anything like that. This is the most important thing, obviously.
The sights were pretty bad, gave me a headache but POA = POI, so again you can't knock them too much.
Ergonomics... I thought they were kind of bad. The slide-mounted safety flat-out sucked and was a pain to maniupulate. Overall, the gun had a blocky, awkward feel.
Accuracy was OK. No bullseye gun, but you could hit man-sized targets at 25 yards with it, so it wasn't obviously innacurate either.
The trigger... HORRIBLE. It stank the place up like a skunk cabbage. The DA pull was the worst I've ever encountered... long, heavy and uneven as hell, and the last 1/4" of travel felt like you were pushing a nail into a 2x4 with your small toe. SA was stiff and gritty, but not as bad.
Overall, the P89 struck me as a workable weapon, but rather crude and not that hot of a design overall. Compared to the CZ-75B that I bought for about the same money, it was no contest... the CZ was a vastly superior all-around handgun. Just as reliable, but with better sights (barely), much better trigger, much better ergonomics, much better fit & finish and more accurate.