I have two Colt pre-ban R6830s. I initially purchased them because they were offered in deals that were simply too good to pass up. Since then I have found them to be quite a fun package and they now definitely enjoy a home in my collection. To date I have only used American Eagle 7.62x39 in them as I bought a ton of the stuff at basically fire-sale prices. Shooting from a bench, I have been able to achieve 1.5" MOA accuracy. Not too shabby for a carbine length barrel and open sights. The close range knock down power makes it fun, too. Pop cans and assorted target really fly when shooing them at the sand pit I tend to frequent. Another plus of the 7.62x39 is that it is a legal caliber with which to hunt medium size game such as deer here in Washington State. I plan on eventually giving mine a whirl at this, too.
As far as magazines, you can use standard USGI magazines. The original Colt 7.62x39 5 round magazines were nothing more than a USGI 20 round mag with an internal block limiting their capacity to 5 rounds. The only catch with using 7.62x39 in USGI mags is that they will only reliably feed 6-7 rounds. This is due to the more extreme taper of the 7.62x39 cartridge.
If one is interested in 7.62x39 AR-15s, I would strongly recommend they invest in some of the appropriate aftermarket magazines as well. What you want are the hybrid type magazines. These are constructed by welding some sort of AR feed tower to some sort of AK magazine body.
Normally I would say that USA magazines are shit, and as a rule they are, but there's an exception to every rule, and the one to this rule is that USA 7.62x39 AR-15 30 round magazines, Model EXC133, actually either work well or will with a minor amount of tweaking. They use a USA Magazine brand steel AR feed tower spot welded to a USA Magazine brand proprietary AK magazine body. It has a follower that operates the bolt stop. I have had good luck with them and others have reported similarly favorable experiences.
CDNN used to have the USA mags for $19.99, not a bad deal. But last I heard, they were completely out of stock and we all know that they ain't making any more of these right now. I'd snap 'em up if they have any more.
My favorite 7.62x39 30 round magazines so far are the Scherer and the Gordon Technology brands, though. I don't know where either of these companies got their steel AR feed towers, but they are TIG welded to genuine AK magazine bodies and use AK magazine followers. These are tough, reliable mags and usually command a premium price. Expect to pay at least $40 each if you can even find them. The only drawback to these type of mags is the lack of capability to operate the bolt hold open feature of the AR. I'm willing to forego that feature for the tough, durable nature of these magazines, however.
One of these days I should just write up a definitive 7.62x39 AR-15 FAQ and have it tacked in one of the forums.