My wife bought herself a PPK/S (S&W/Walther) last summer. It's a very nice gun. I had no real interest in it initially, it's turned out to be a pretty cool little shooter. I have no direct experience with the Interarms versions, but IIRC, they have a good reputation.
As I understand things, the frame on the PPK/S is different from the original PPK. I believe it is slightly bigger to prevent it from being catagorized as a dreaded Saturday Night Special. I could be wrong, but that's how I remember things.