How much does that affect a user? I mean, if you take the time to learn how much it strays, are you at that point just as accurate as someone with a more accurate weapon?

I've never used a gun so I'm curious.

All rifles do this a bit; snipers/designated marksmen get very accustomed to their personal weapons, and need to take time to re-acclimatize with any other rifle. For a regular line infantryman in a modern army who is mainly using his rifle for suppressive fire it doesn't significantly reduce his effectiveness.

I'm guessing this is why the Riflemen's Creed (http://en.wikipedia.org/wiki/Rifleman%27s_Creed) is so important?

I'd say indoctrination plays a part as well.

In real close-to-mid-range combat, this matters even less (unless you're a marksman). It's rare that you'd have time to properly line up the sights and take one really good shot. What really matters in that situation is knock-down power, reliability, and the shooter's training.

They are also stupid easy to manufacture. The design has been copied numerous times as well. Basically, they are the Honda Civics of military weapons. Cheap, easy to use, reliable, easy and cheap to fix when they do break, and lasts forever.

I've seen AKs in 3rd world countries that were probably less than 20% original parts.