You've seen them online or in an NRA publication. You have seen a few reviews of these rifles on YouTube. There are numerous manufacturers that have started producing them. ARs chambered in 7.62x39 are here and they are selling. The question is: should you spend your hard-earned money on one?
This could be a very short article - If you don't have an AK, why would you buy yet another caliber that you'll have keep In stock and new magazines on hand in order to be able to use the gun? (If you have an AK, do you really want one of these?) But, there is more too it than that. There are deeper issues with ARs chambered in 7.62x39.
Wait, what's wrong with 7.62x39, you ask? Nothing, really, but it's not my first choice... or even my fourth. It's not Russia's first choice either: they mostly shoot 5.45 these days. It's not a bad round, it's just not optimal. Basically, you're investing in a round with limited US production and limited choices for quality loads in brass cases. To make matters worse, outside of trying to shoot through a car (a heavier bullet has it's trajectory effected less when passing through barriers), this round has no real advantage over 5.56. It's heavier to carry, bulkier, less available, has fewer high-quality precision options, worse drop over distance, and it's not any cheaper than similar quality 5.56. If you value SAWC, as I do, 5.56 is clearly better. 5.45 is much more interesting than 7.62x39 to me, as well.
But, as I said, this problem runs deeper than the ammo. These ARs don't have AR-style magazines. Instead, they use AK-style magazines and there are compatibility issues between various manufacturers. They also have had to redesign the lower to accept these AK mags, which is likely contributing to the problems with feeding that have popped up in reviews of these rifles. Basically, companies like CMMG and Rock River are taking the biggest reliability issue with AKs and mating that with an AR. Why are they doing that, you might ask? Simple: AK mags exist, so it decreases production costs and allows consumers to use mags they already have or are widely available on the market. But, the slight differences by all the various manufacturers, in all the various countries, make these mags a crapshoot when it comes to finding ones that feed reliably in your specific gun. This problem is magnified by the fact that the bcg is similar to an AR, not the AK platform these magazines were intended for.
Even if you can get past these issues, there's another, perhaps more obvious issue: if you want a gun chambered in 7.62x39, why not just get an AKM variant rifle? Why would you want to get an AR that loses many of the advantages of the platform? (After-market Manufacturing, widespread availability of parts, easy to load mags, reliable feeding and ejection, etc.) It's got almost all the problems of both platforms.
The only reason I can think of to get one of these rifles is if you have $1500 lying around and you already have AKs - then, this makes a great range toy. For practical purposes, there are better rifles out there. These rifles are something you should skip, if you aren't looking for a range toy.