Walther PPK/S are great guns! My buddy also has a Walther P-22 lol such a fun plinker since it shoots 22lr.
The M9, as a whole is a very good package. It's just the feel of it, from a 1911 that had me all up in arms about it. The trigger pull, and grip just felt "wrong." Beyond that, a buddy of mine, a Marine who spent time in the dustbowl said it was terrible because of its unique slide. It'd get crud and grime into its slide within the first 20 seconds out. I believe that is the main bane of that weapon, so at home, it's ok.
The H&K mk23/USP felt enough like a 1911 that it made it easier for me to adapt to, and after ten years of using them, I'm sold on those bricks. But still, it's the familiarity, like in all things, that has me sold on it. Within my first day, I could field strip it, clean it, and get her back together pretty much blindfolded because it was so similar to the 1911. The catch, latch, saftey, even the barrel, are all very similar to 1911s.
Gotta say though, this is hands down my favorite pistol of all time:
with a cartridge conversion, nothing is better to me.
I like the pocket models too, even though they're small in caliber, I'm pretty sure at 5 feet the perp would still count it. After seeing what I've seen a 22lr do, I'll put faith in a .32 at defense ranges. It helps to be able to aim too