A guy I used to work with had the Kel Tech.
He liked it as it took S&W mags and since it folded in half it fit in his lap top case.
So depending on where he was headed he had a small carbine with him. He was carrying it with him when the OJ jurry came Back.
So he had it for a number of years and likes it. I think he had problems with the larger capacity 30 rounds after market mags, but all of the S&W factory mags worked like a champ.
Also had a few friends hat had Ruger PC-9 carbines and they too liked them alot.
I think they would have caught on more if Ruger would have made a few different magizine wells to take the major brands of pistol magazines.
LAPD looked at them and wanted them to take the Beretta 92 Mag to match the issue handgun. Ruger said Issue our handguns rather than make a lower mag well to take Beretta, Sig, Glock, or S&W. Had they done so.... LAPD would have had them in the cars in time for the West Holly wood shoot out and there would not be a bunch of AR 15's in squad cars now all over the country.
You would see Rugers that accept the mag of the issued handgun. In the Case of CHP S&W, NYCPD would be Glock.