I bought a PPK by Interarms to see if I can see what the quality differences are compared to my S&W PPK/S. Externally I don't see many differences that matter to me, though many don't like the way the laser engraving of the S&W logo is, or the way the serial number was done.
Internally it appears to me that the Interarms has cast parts where the S&W has MIM parts. If I had a choice I'd personally choose cast over MIM, that's just me. The frames on both appear to be cast.
I haven't shot my PPK from Interarms, as it's NIB and I'll keep it that way.
The PPK/S works very well, is accurate and reliable, in short, everything I like in a gun.
S&W made some changes to the safety internally, I don't know if that was because of the change to MIM parts, but obviously they've had troubles.
I like the fact that they were both made here, and those who favor German or French made guns discount that. The whole concept of hammer drop safeties and double action are mistakes in my opinion, but it is what it is and seems to work well. If I didn't have a PPK/S I'd look for an Interarms one because it's stainless and has better parts than the S&W, but the price difference is getting out of hand. Apparently a NIB PPK/S by Interarms is around $800 if you can find one, and S&W's aren't that much worse.