I've owned the licensed S&W version for quite a few years (SW999c). Walther either made or provided everything except for the bare slide and barrel.
I picked it up as an armorer just as it was being phased out of production in favor of the SW990L's (what Walther calls the QA sear action, except the S&W version lacked a field-stripping button in the slide). I thought about ordering the P99c AS at the time, but the SW99 had a few features I preferred and it was priced just a little less than the P99 (since S&W didn't have to buy the slide and barrel from Walther for the S&W model).
I've always thought that Walther ought to have supported more advertising and promotion of their 99 series in the US when they had their
Strategic Alliance with S&W, and used S&W/Walther America as their importer.
S&W did a little advertising and support of their own licensed versions, mostly for LE, as I recall.
There have been any number of revisions, refinements and improvements to the 99 series over the years, and in my opinion they're very under appreciated. They've seemed to have been real sleepers on the American market.
FWIW, my SW999c feels slimmer in the hand than my pair of G26's, and I can't use the smallest of the 2 backstrap inserts because it makes the grip feel too small (I have medium-size hands, despite what most glove makers seem to think).
The AS triggers seem to smooth out and lighten up over use. The older guns with the "ramps" inside the bottom of the trigger guards could result in some trigger finger soreness if you let you trigger finger drift too low (especially in cold weather), bumping against the ramp under recoil (in the .40's), and I always thought the backstrap inserts for the compact frames were more ergonomic and comfortable than those used on the larger standard frames. That's just personal preference, though.
I'd think it was worth taking the time to try and find one to look at and compare.
BTW, Walther used to only offer a 1 year warranty on the P99's, but a limited lifetime warranty for American owners of the then-new PPQ. I'd heard Walther was now offering a new limited lifetime warranty through their new Walther Arms company (their own US distributor) on their other guns. That would be a welcome benefit. It would be enough to make me consider making sure I was buying a gun that was covered under the new warranty.
http://www.waltherarms.com/legendary-lifetime-warranty/