A trip to the Kel-Tec site showed that the SU16 is offered in a variety of models. There are five of them in all by my count. The lineup starts off with the SU16A, and is followed up by the: SU16B, SU16C, SU16CA and SU16D. The main differences between these models seems to be barrel length, type and location of front sight, and type of stock.
The SU16A rifle is a gas operated, semi-automatic, chambered for .223 caliber (5.56 mm NATO) . Some of the features of this rifle are: Black Polymer stock, integrated Picatinny rail, chrome lined bore and chamber, the fore-stock can fold down to form a bipod, the buttstock can hold two spare magazines, and the butt stock/trigger group can be folded forward to make the gun more compact for carry (non-firing in this mode), it is compatible with Ar15/M16 magazines. The barrel length is 18.5 inches. There is no mention made, on the Kel-Tec website that I saw, about the sights being adjustable but my guess would be they are. Unloaded weight is 5.01 pounds. It comes with 10 round mags. More info here.
If you want something with a shorter barrel they offer the SU16B with a 16 inch barrel. The website specifically states that this one has adjustable front and rear sights. The front sight adjustable for elevation and the rear for windage. Unloaded weight is 4.51 pounds. More info here.
The we move onto the SU16C. Here we move away from the folding buttstock/trigger group as this one has a conventional folding stock. When the stock is folded under, the weapon is still capable of firing. It too has a 16 inch barrel and most of the features of the above two rifles with the added features of a dust cover and case deflecting operating handle. This one also moves the front sight back from just behind the muzzle to near the front of the fore-grip (integrated with the gas block). It weighs in at 4.71 pounds unloaded. More info here.
The SU16CA goes back to the folding buttstock/trigger group. It has all of the operating features of the C model but uses the stocks of the A model. More Info here.
Finally they offer the SU16D. This one is quite a it different if only because of the 9.2 inch barrel length and the 3.71 pound weight. It utilizes a conventional folding stock (fold under) allowing the gun to fire with the stock folded. This one looks like a real Zombie Killer to me but alas I will never be able to buy one with a 9.2 inch barrel here in NY. More info here.
If I was going to get myself one of these rifles I think for practicality's sake it would either be the SU16B (pictured up at the top) or the SU16CA (pictured to the left). I am not too sure I like the shorter sight radius of the SU16CA as compared to the SU16B but I could probably live with them and I like some other features of this model over the SU16B. I like certain features of these rifles such as the fore-stock being able to fold down to form a bipod and the buttstock holding two extra magazines. If the rifles are made to a standard of high quality I imagine these features would be great; however, if quality control is lacking I could imagine magazines falling out of the stock and the bipod shaking like Jello. As I said, I have not tested these rifles but I should also point out that others have done so. Here are links to three such tests: http://www.equipped.org/Kel-Tec_SU-16_Review.htm, and: http://www.ktog.org/cgi-bin/yabb2/YaBB.pl?board=105%3Baction=display%3Bnum=1107839748, and: http://www.ammoland.com/2009/03/26/kel-tec-su-16-223-remington-ar-rifle/ . The guys who tested them seemed pretty impressed in a good way but you can read the articles and see that for yourselves. Here is a link to an online gun forum wherein the users have rated their own SU16s.
Gun Tests Magazine also rated this gun in a comparison with a Springfield Armory SOCOM. Their results can be seen here but only if you are a subscriber. I will only say they gave it a Buy It rating and that the ammonland link above will pretty much show you their report on the SU16. (Shame on me for allowing my subscription to this magazine lag a few years back. Good for me, I just subscribed to it again. A great deal for $20 per year with paper magazine subscription and online availability of their articles for current and past gun tests.)
I would think that if I was in the market for a semi-auto in .223 I would consider the Kel-Tec. Of course, if I wanted something in .223 I would also consider a Ruger Mini-14 which in my estimation is probably a somewhat better made rifle especially now that Ruger has refined it in the past few years; my friend seemingly thought so too - he bought a Mini-14. Still though the SU16 is an appealing rifle to me, and it would make a great packable rifle for a grab and go kit or for a backpack. It would likely be a good survival gun, certainly in a caliber that could handle anything from varmints up to deer and then also be used in combat.
All the best,
PS: They apparently have pretty good customer service at Kel-Tec, see this.