Millions have been built over the decades. They're simple, completely reliable, cheap, and have good firepower. Perhaps their only drawback is that they aren't the most accurate weapons at ranges over a couple hundred meters. However, they were damned near ideal for the jungles of Vietnam because they almost never jammed in the harsh tropic environment and long range fighting was very rare. They were definitely better than an M-16, IMO, especially in close range fighting.

I trained on the M-16A1 when I was in the infantry (75-77). It was hard enough to keep the damned thing in working order at Fort Polk, LA, much less under combat conditions. I've been told the M-16A2 doesn't jam as often due to the chromed receiver, but I'm still hearing soldiers complaining about jamming due to the fine grit sand in Iraq. There's a new infantry weapon in development (a combined 5.56mm selective fire rifle with a 20mm grenade launcher). One can only hope it proves more reliable than the M-16. At a cost of about $25K each, it damned well better be more reliable.