I think it all depends on what form of PPK you choose.
By that I mean, if you are buying a German made gun, say a pre WW2, then the PPK is an classic handgun - and fun to have for history's sake. They are beautifully made things -sculptures really, and to me seem about as near as you can get to the "perfect" handgun.
Post WW2 PPK's, not quite so finely made, will always be associated with James Bond, still classic pistols. I have never found the original German PPK to be particuliarly available. In years past the post war pistols always seemed the hardest to find. Now, with the internet more are available for sale than ever before, but at pretty high prices.
If you are wanting a pistol to carry or shoot, and go Post 1968, then the PPK/S actually was a good choice. With the full metal backstrap of the PP, in the U.S. preferred .380 ACP, it is far easier to shoot than the PPK. The PPK/S is a bastard combo of the PPK slide and PP frame, created to get around the 1968 GCA which banned the PPK in the U.S. With the flat magazine installed it is only a 1/4 longer than the PPK.
For shooting/carrying PPK/S is the better choice - stronger too since if you were to drop a PPK with the plastic wraparound grip, it might shatter and the gun would be impossible to shoot!! If you choose the PPK in .32 or .22, its a fun shooter and just plain neat! They are definately marginal for self defense, but worth owning nonetheless.
In the Interarms made pistols, they are still faithful to the original design and well made. I would say the equal of any Walther produced as "shooters". In the newer Smith & Wesson, IMO they begin to look like Chinese knock-offs of real Walthers, with the extruded beavertail, thicker grip and the unecessarily sharp and unfinshed bevels on the front of the slide. Quality of the S&W appears uneven at best.
So in the modern shooting pistol, go with the stronger PPK/S especially in the .380, there is no real advantage to the PPK - and more disadvantage with the weaker grip.
But when looking around for that next Walther don't forget the PPK's big brother the PP, still available as surplus at reasonable prices and of great quality -everyone who loves Walther's should have one. If you go stalking the originals, it is a worthwhile hobby, but carry a lot of green - you'll need it.