The 7.62x39 is the same round that the AK-47 shoots. It's shorter than the 7.62x51 NATO, 7.62x54 (Russian rifles like the Nagant) and the 30-06, although all of them shoot .30 caliber bullets. I consider it to be effective to 2-300 yards.
The advantage of the SKS USED to be, it's dirt cheap, and you can beat it up. Now it's somewhat cheap and you can beat it up. (I got mine for $100.) You can strip off the cosmoline with brake cleaner, (which I would never use on an AR-15,) and drag it through the firt, leave it in the back of the truck, give it a cursory cleaning every once in a while, and not feel guilty. It shoots a potent .30 caliber round, id accurate enough for combat, and it's so reliable it's boring. The ammo, like the rifle, is cheap, but not as ridiculously cheap as it used to be.
There are accessories, but be careful. First of all, there are regulations about what you can and can't do to it, and in what order and quantity. We abbreviate this to "922r". The SKS has a special import status, and if you modify it, there are guidelines you have to follow. Higher capacity, magazines are available, but their reliability ranges from mostly ok to don't even try it. (Mostly don't even try it.) There are aftermarket stocks, which are mostly a good idea. You can't get worse than the original stock. (They were designed for muppet pygmies.) There are muzzle brakes which replace the grenade attachment on the Yugos, I haven't tried one, but I might one day just for poops and giggles. There are scope mounts, but most of them replace the rear action cover, which isn't held on very securely, and you must remove it every time you clean. (If you decide it's necessary.) Which means they won't hold a zero. You can get scope mounts that require minor surgery to install, but they are more expensive, and you have to pay a gun doctor for the surgery. The best add-on I've seen is tech-sights, which cost about $65, and mount on the rear of the receiver, (giving you a much better sight radius) and changing it to an M-16 type sight picture.
I would use it for white-tails, but not mule deer. Might be perfect for javelina and other small-medium sized game. I think it's the perfect truck rifle, it's tough and effective for emergency combat, and if you bump into a cop who doesn't know the law and decides to confiscate it, it's easy to replace.