So I've made a couple of gun purchases lately, both in .223. One an AR and one a Keltec SU-16c. Out of the two I was surprised to find that I love shooting the Keltec far more than the AR, but both have been boringly reliable.
Let me preface this by saying that I don't think the Keltec is a substitute for an AR or any other proven battle rifle system, but it is a very useful and interesting gun. The Keltec Su-16 series incorporates elements from both the AK and AR system, the basic bolt carrier setup of the AR (there are a few differences), and the piston system of the AK.
Disassembly and cleaning is a breeze once you get the routine down, and no tools are required, as it should be. I have put 500 rounds downrange now, and other than swabbing out the barrel have not touched it since the original cleaning. The chamber and bolt carrier are exceptionally clean for that amount of rounds, almost no grime whatsoever. Accuracy has been acceptable, 1 inch groups as 25 yards, 1.5 inch groups at 50 yards, and 2-3 inch groups at 75-100 yards. This is using just irons and a variety of cheap ammo. Reliability has been fine, had one fte early on when switching from steel ammo to brass, but the gun did run the steel ammo fine before switching. Durability has yet to really be tested, I haven't hurled the gun to the ground or tossed it into a mud pit, but it does ride in the trunk of my car with no case and has been on a few backpacking trips and has held up without any sign whatsoever of wear.
Ergonomically it performs well, length of pull is acceptable, and the folding butt stock is surprisingly sturdy. Mag release is where it needs to be, but the safety is awkward as it is slightly behind trigger. The folding bipod is interesting in concept but wobbly in real life, that said, if no other rest is available it does do it's job fine. The guns compactness and light weight make it just a joy to carry around, that alone is worth checking it out for.
Overall my opinion is very favorable, for the money it's well worth it. Accurate enough to do most things, reliable enough to be used for defense, durable enough to not worry, and cheap enough to use it as a truck/trunk gun.