I was looking VERY seriously at a 7.62x39mm AR, but the problems I found were as follows:
-One of two FTEx problems: the coating sticks the case in the breach, or the non expanding steel case allows carbon fouling to build in the chamber, and sticks the case
-Special bolt assemblies better handle steel cased, but are only available in the states (read: fun with exports!)
-Mags aren't great (CProducts is coming out with a stainless AR mag in 7.62x39, but you'll have to wait until after ShotShow and until someone brings them into Canada)
But the ammo is cheap, and it would still be a fun gun to have!