I just got one earlier in the week..haven't even taken it to the range yet, but I have a full-size P99.
Mags seem a bit pricy compared to Glock mags, $35 or so...but, I expect them to be just as reliable.
You can get spare parts from Walther, usually fairly cheap, so there is that. Waltherforum has PDFs listing all the part numbers, so you can just call and order by that. If you can only find flat-bottom mags and want to put a finger rest on it, for example, you can order finger rest floorplates direct from Walther.
Full size P99/PPQ M1 mags fit just fine, and you can find mag grip sleeves at Midway and elsewhere, though the stock of them seems to come and go...and at $20/ea, they're kind of pricey. But, if for some reason you want to carry a subcompact with a 17 round mag...you can.
You can improve the DA trigger pull by replacing the striker spring with a 6lb XP Glock striker spring, supposedly. I'll find out today when I get home, as my spring set should have been delivered today
Even without that, the DA pull is pretty nice, way better than a factory Sig P229 for example (my only real comparison, as that's the only other DA/SA gun that I own at the moment). It's so far the only DA/SA platform that I can actually use, that works for me.
Jarvis sells threaded barrels for them, if you feel the need to silence your subcompact...I know I do, I'll have one on order in a couple months when I can afford it
I also have a PPS also to compare it to...it's just about the same size as the PPS in every dimension save one - it's wider. In every other way though it's just about the same size (though I think it's a smidge longer in the slide).
P99/PPQ sights fit on it just fine...eventually I'll be ordering some TruGlo TFX for my P99 and P99c.
This weekend I hope to get to the range with it, and if it works like I expect it to, it'll have a place in my carry rotation...