I've been using and owning SW99's for about 10 years now.
I've been an armorer for them for about that long, too (3 classes).
I carried an issued standard model SW99 .40 for a few years and I own a similar standard size .40 and a compact 9mm (both in traditional double action, what Walther now calls their Anti-Stress trigger).
I think the 99 series has some worthwhile refinements incorporated into the design when it comes to a plastic-framed service pistol.
Most of the parts I've had to replace in SW99's (and the occasional P99) have been Walther produced/provided parts.
I happen to like the design changes S&W requested in their SW99 frames.
S&W uses the optional (heavier) extractor spring in their models, and reverses the orientation of the spring in the slide. The stronger 'hooked' slide stop lever spring is easier to unintentionally snag and damage in the SW99 series than in the P99 series (which uses a shorter spring with a closed loop on the end instead of an exposed 'hook').
Like the Glock, I happen to prefer the 99 series when chambered in 9mm.
All things considered I like my SW99's quite a bit, favoring the compact 9c model.
I also happen to think the new M&P series contains some further improvements and refinements when it comes to plastic pistols, but I'm not getting rid of my SW99's.
