I have an Interarms PPK/s (slightly longer grip) with about 1500 rounds through it, plus 400 from a previous owner.
Pros:
-All steel (I like having the weight to soak up recoil)
-Small, but not too small. Its about the smallest gun I'd ever have, because I like having some gun to hold onto to get a proper grip and bring it down for follow-up shots
-Good single action trigger. A little heavy, but very little take-up and breaks cleanly. Decent reset, too.
-Accurate. I can hold 4-5 inch 50 shot groups at 25 yards if I slow down a little, and from the way its grouping I can tell that at this point I'm still the limiting factor, not the gun.
-Very slim, conceals easily. I carry mine either in a shoulder rig under a dress shirt or appendix carry. Very comfortable with no printing. Also fits in pockets if you wear loose pants.
-Good ergonomics. Very natural fit in most people's hands. Easy, natural reach to mag release.
-Extraordinarily easy to field strip.
-Boat loads of fun to shoot. I know this is horribly subjective, but shooting it always puts a smile on my face.
-Very, very pretty.
Cons:
-It'll bite you. High grips or meaty hands will get lacerated by the slide rails. A little experimenting will help you find a good grip that lets you control recoil without having to wash blood off your gun after every range session. This is not an issue if you get one of the S&W ones with the big beavertail.
-More recoil than you'd expect from such a small caliber in such a heavy (for its size) gun. Again, shooting it a lot and experimenting with different grips will land you on one that's comfortable.
-Can be difficult to rack slide. The recoil spring is very stiff, and the location of the safety in the middle of the slide serrations can make it difficult to get a good grip on.
-Can be finicky with hollowpoints. I carry ball in mine, as it doesn't feed Hyrda-shoks well, and I haven't been able to get my hands on enough of another brand to do any proper testing.
-Destroys recoil springs. I'd recommend having several on hand and replacing every 1000 rounds.
Matters of personal taste:
-Slide mounted safety works backwards from most American guns (safe is down, fire is up) and some people don't like that. It doesn't bother me because I carry it decocked with the safety off.
-Heavy double action trigger. Most people seem to hate mine, but I like it more than most double actions. It is very heavy, but all the weight is in the first part of the trigger pull, so once you get past that, the trigger sort of snaps back the rest of the way. I find it very easy to pull the DA trigger quickly without moving the gun.
-Small sights. Doesn't bother me too much since the rear line/front dot is really quick to acquire for me, but many people don't like them. What I don't like about them is that they're unfinished metal (on my stainless gun at least) and can be hard to pick up sometimes. I'm going to fix that with some flat black model paint.
-No external slide release. You have to slingshot the slide to release it from lockback, and you can only lock it back with an empty mag.
Notes:
I've vastly improved the shootability of mine and eliminated recoil discomfort by putting a Houge grip sleeve on it. The palm swells make it easier to get a good grip on, and by positioning the finger groove you can force yourself to always grip it in a fashion that won't get your hand cut. It makes the gun wider, so you have a trade-off in concealability, but I find it worth it, especially since it distributes the recoil force over a wider part of your hand. (that backstrap is pretty narrow)
Cylinder & Slide does custom work on PPKs. Trigger jobs, checkering, even cutting the slide for larger and easier to aquire sights. Here's a
link.
Because its an older design and fairly small, there's a lot of little complex bits you can't really get to to clean. The easiest solution is to have some brake cleaner on hand to hose out the places you can't reach.