Is it worth it to put a 762x39 on an AR?
What are the pros and cons?
I guess that all depends on your use. Cheap to shoot & larger projectile over 5.56, obviously doesn't have the distance capability of 5.56. Some of the cons are that they are picky about mags and the mags are a little more expensive (ASC mags seem to be the most reliable and worked well for me with no feeding issues, run about $15-17). Some people don't like the shape of the mag....
The bolt lugs have been know to fail due to less material on the lug face to make room for the X39 cartridge, but there may have been improvements since I owned one (never had a bolt fail on me, but didn't run high volume).
They will eat steel case ammo all day long, since the primers are harder on steel case, a standard trigger is recommended