At-a-glance information:
Caliber: 7.62x39mm (Russian M1943 cartridge)
Action: Gas operated (Very similar to the AK47-series rifles)
Overall length: 1022mm
Barrel length: 520mm
Weight: 3.86 kg empty
Magazine: 10 round internal magazine
An SKS is is one of the only semi-automatic rifles legal for purchase in California, and thus is very popular. However, some Yugoslavian SKS rifles have grenade launchers affixed to the end of the barrel, which make them illegal. Removing the grenade launcher will return the rifle to a legal status. It is also illegal to replace the SKS's internal magazine with a detachable one, as is very popular in other states.
Late in their production, SKS rifle barrels were switched from using a screw method of attachment to the receiver to a pinned method of attachment. Consensus is that the pinned models are slightly less accurate and thus are slightly less sought after than the rifles with barrels screwed into the receiver. Practically, other factors such as wear and how well maintained the rifle was end up affecting the rifle's accuracy far more than the type of barrel attachment. However, for those looking for a rifle just to use for blasting away, none of those factors will make much difference when compared to the price and availability of the rifle.
Other interesting variations of the SKS rifle include parade rifles, which often are painted white (rather than just being varnished for protection) and chromed or otherwise polished, and paratrooper models, which have a shorter barrel and feature a different style bayonet.
Replacement parts are readily available for SKS rifles. Receiver covers, springs, gas pistons, handguards, stocks, and more are generally easy to come by. Trigger parts or sights are harder to find, sometimes requiring the purchase of a second parts rifle to extract them from. Ex-military rifles from this era are not often found in excellent condition, and thus most SKS's are missing some bluing or have other damage. Scope mounts are also available for the SKS, however, replacement receiver cover scope mounts are often very inaccurate. SKS rifles are easy to maintain, and if treated well, will last for ages due to their thick receiver and chrome lined barrel.
References:
http://world.guns.ru/rifle/rfl01-e.htm
Personal experience