Ok, quick comparison, which is somewhat off since I don't yet have my own ps90, (mine's on layaway).
As they come from the factory, the m17 is longer and heavier. The m17 has a full alloy upper, with a polymer lower. The trigger in both can stand work, but it can be improved with some work.
The M17 comes from the factory with a basically useless set of irons set in a polymer carry handle that has a picitanny rail.
The m17 takes standard AR mags, though I'm nor sure that a beta or a drum will work. It's as accurate as an AR and you actually get more velocity than a standard carbine as it comes with an 21 inch barrel for a length of ~30 inches. I've never had a failure on mine, it's a piston gun.
Mine was modified by Kurt's Kustom Firearms, the barrel was cut down to 18 inches making it 26.75 inches long, the carry handle was removed, and replaced with a pictanny rail and the charging handle changed to a folding one. The receiver was shortened and lightened somewhat too. The pic of mine is here:
http://www.kurtskustomfirearms.citymax.com/albums/album_image/42606/42381.htm
Anyway, I'm curious about what negative stuff you've heard. It's not been a bad gun, even for what I have in it for custom work. I've occasionally run across used ones for ~550 and they are a deal for that price.