I prefer the AS (TDA) trigger to either the QA or DAO, but then I prefer TDA triggers in my regular working pistols ... and Walther designed and produces a good one in the 99 series. I realize that not everyone prefers a TDA action, however, and the QA action is a nicely smooth one for its type.
While I've had excellent service from my personally owned SW99 .40 S&W, as well as in the similar issued one I carried for a few years, I happen to feel the 99 series pistols are at their best when chambered in 9mm. Just my druthers ...
I sometimes have to chuckle when reading fervent enthusiast's postings, though, especially when extolling the virtues of one manufacturer over another when it comes to similarly high quality firearms ...
FWIW, the greater number of broken or failed parts I've had to replace in SW99 & P99 pistols have been parts which came from Germany. That's why manufacturers produce spare parts, you know.
The last sear housing block I had to replace, because of a broken ejector ... and which is a German part ... was in a SW99 .40 through which the owner told me he had exceeded 50,000 rounds fired. I've had a couple break off sooner than that, though (just as with some other make/model pistols).
The Walther P99 is an excellent pistol, and the licensed SW990L is an excellent pistol, but I'm glad I bought both mine when the SW99 TDA's were still being produced for commercial sales (instead of only LE sales, as they are at present).
Yes, night sights are available.