Personally, I have a strong dislike of the M16/AR-15 platform, so I speak with that bias: but I prefer the Kel-Tec SU16 to the AR15 in many ways.
1. It's got a decent gas-piston action, which means that the AR-15's ghastly direct-impingement gas system is no longer an issue.
2. It's much lighter than most AR-15's. Given the minimalist nature of the 5.56mm. cartridge, a lightweight weapon is, to my mind, a much more sensible match for the round.
3. It folds for ease of carry, storage, etc. The SU16C model can even be fired while folded. This isn't important for most uses, but does help when travelling (it's a wonderful carbine for suitcase carry), and if I have to deploy it in a hurry at close range, it's comforting to know I can fire it while folded.
4. It's very accurate for so low-cost a weapon. I've found many brands of ammo will give solid 1-2 MOA performance all day long. The Mini-14, by comparison, is typically a 3-4 MOA gun at best. A decent AR-15 will be as accurate as the SU16, and a top-of-the-line AR even more so, but at twice the price or more.
5. I don't like carbines or rifles with a whole lot of hardware bolted on to them. I'm not going to be carrying a rifle with an attached flashlight, vertical foregrip, laser designator, coffee maker and hairdryer, as seems to be the case with so many "tacti-cool" AR-15's. The Kel-Tec is a light, handy weapon that carries very easily. Just what I want!
Thus, of my four 5.56mm. carbines and rifles, two are Kel-Tec's of various models.