Over the years I've had 3 Marlin Camp 9's. One new and two I bought used. The last one I bought at the Houston Gun Show for $300.00. That was last summer.
It has been my experience that these are well made, dependable rifles. High capacity magazines are everywhere and folding stocks if you wish. Mine wears a red-dot or a laser and is a great plinker and walking around gun. Also a very nice home defense rifle.
Mine have always been reliable, well made, and retain their value well. It is a much more traditional rifle than the Kel-Tecs and doesn't look "black and scary" like them and the AR's or Hi-Points, if this matters to you. To the casual viewer the rifle looks much like a .22.
My Camp 9 lives on a steady diet of CCI Blazers, lead reloads, and any other El-Cheapo 9mm I can find. I feed it Winchester Silver Tips if I'm going hog hunting with it.
Good luck with your 9mm carbine quest and let us know what you decide.