I've had a few and put several thousand rounds through them, they're great guns. Their build-quality is on par with H&K and from grip-shape to materials to finish they are head and shoulders above what most of us are used to from Glock, XD, or an M&P. The grip-angle and overall ergos are fantastic and up there with the classic Lugar, 1911, Hi-Power, and CZ-75 in how well it feels and works.
IMO their only real drawback (and just could be for ME and my grip) is that their slide-release lever is actually overly ergonomic. If you're in the camp that likes to install the extended slide release on your Glocks, than you'll probably love it as it's a better shape and is easier to activate than any available on a Glock, however and that said, if you're like me, and you use a high-thumbs-forward-grip, you'll probably end up riding it and end up not having your slide lock back on empty mags... this is one area where I wish they hadn't tried to improve as IMO the stock Glock design is in-reality better in-practice for more advanced shooters with more training as it's quite hard to contact accidentally, yet still provides enough of a platform and leverage to drop the slide on a fresh mag with one's strong-hand thumb during a speed-load (with practice of course). The other minor thing is that the older ones had a "C"-shaped washer that held their striker-springs captive in their striker-channels which would produce an ugly feeling and heavier pull now and again when the washer landed in a clocked position, the newer M-A1's being imported over the last year or so have been upgraded and improved and do not do this.
Otherwise and besides those two things, especially with the newer improved striker-spring channel, they may very well be the best striker-fired gun ever produced, definitely one of the best.
They are getting harder and harder to find as steals, if you are going to pay more than $500 I'd recommend trying to get one of the latest specimens (but my guess is that they'd upgrade an older one without question, their customer support is just awesome, their head armorer Jeff rules and literally took care of legions of Steyr owners for a few years while they weren't being imported and I'd doubt you could find a single negative word out there at all).