The Kel-Tec SU-16 is quite popular in California, where it was (until the advent of Off-List Lowers) the closest thing to a true detachable-magazine AR-15 that a post-2000 rifle buyer could get.
For Californians, it's worth noting that the SU-16CA is widely considered to be the most desirable model. The earlier SU-16A and SU-16B models featured a somewhat flimsy fiber-optic front sight post.
All of the SU-16 line is gas piston-driven, as opposed to direct gas impingement.
There are a few rumors about the SU-16 which are completely incorrect, and I'll try to list them all here. It's fairly easy to get the wires crossed on these, given that the parts actually look quite a lot like AR-15 parts. Someone who didn't have both rifles next to each other for comparison (like I just did) could easily draw the conclusion that the SU-16 actually is using AR-15 parts.
The SU-16 uses mil-spec M-16 / AR-15 firing pins: This is absolutely not the case. AR-15 firing pins will not even fit into the SU-16. It may be possible to turn one in a lathe to cut it down to fit the SU-16, but the SU-16CA I have absolutely will not accomodate a stock AR-15 firing pin.
The SU-16 uses an AR-15 bolt: This is partially correct. Were an AR-15 bolt to be cut at a certain spot, I am confident that it would function in an SU-16. However, an unmodified AR-15 bolt absolutely will not fit into it.
Universal Features of the SU-16 Line
5.56mm chamber: This allows the rifle to fire either 223 or 5.56mm ammunition.
Polymer frame: The SU-16 line is made of polymer. This is why the rifle is so light and inexpensive to produce.
1:9 twist barrel: SU-16 barrels use a more or less standard AR-15 twist rate. The only thing which might be desirable here would be a 1:7 twist to stabilize heavier bullets, such as the M-4's harder-hitting 77gr load. As it stands, the SU-16 can easily stabilize up to 62gr bullets, although 69gr MAY work, depending on conditions and how hot the rounds are loaded. A stiff load can make up for a little bit of slow rifling.
Chromed OR non-chromed barrels: The only model of SU-16 which has always been chrome lined was the SU-16D. However, from some point in 2008 onwards, Kel-Tec started using chromed barrels in some of their rifles, although it doesn't seem to be all of them. Thanks to Pete Eriksen for noticing the universal exception for the SU-16D.
Folding stocks: This allows the SU-16 to be stored in a shorter configuration than its standard. It's worth noting that, on all but the C/D models, the stocks are not considered folding in California because the rifle cannot be fired when the stock is in its folded configuration. The C/D models use a different stock which is designed to allow the gun to remain operable while folded.
Integrated bipod: All SU-16 models other than the 'D' variant have a built-in bipod mounted over the barrel.
A Breakdown on the SU-16 Line
SU-16A: The 'A' variant is the earliest model currently available. Its barrel length is 18.5", and it features the much-maligned SU16 fiber-optic front sight.
SU-16B: The 'B' variant is the same as the 'A', except for a shorter (16") barrel. It also uses a standard AR-15 front sight post. The standard front sight is also available as a retrofit for the 'A' model.
SU-16C: This is the only non-NFA-regulated SU-16 rifle which cannot be legally purchased in California (due to assault weapon status). It's functionally the same as the CA, except for a folding stock/pistol-ish grip. Other modifications involve the front sight (AR-15 spec, as in the SU-16B) being relocated from the front of the barrel onto the gas block. The lack of a front sight blade greatly reduces the chance of the rifle snagging on things. The SU-16C also features a threaded barrel, which allows it to accept AR-15 muzzle accessories.
SU-16CA: This is basically just an SU-16C with the non-firable folding stock from the A model. The model designation of 'CA' is supposedly derived from this, but it just happens to be the most appealing California-legal model of SU-16.
SU-16D: This is an SBR version of the SU-16, with a standard M-16-like handguard. It's available with 9" or 12" barrels.
PLR-16: This is a pistol version of the SU-16. Naturally, it's not California-legal.