I've used the armour-as-DR system in Ken Hood's grim-n-gritty HP rules (or maybe a variant thereof, been a while). Worked decently enough, but there were some burps concerning types of damage reduced, or whether called shots to avoid armour (to deliver poison with a blowgun, for example) were OK. Like any system designed to be a bit more realistic, you run into some simulationist headaches.
I've read the UA version and didn't like it as much, the DR values seemed too low to be worth the trouble. I've also read the OGL Ancients version, in which there's a coverage roll to see if armour acts as DR versus some damage, takes some of the damage itself (damaging the armour and eventually making it less effective), and then the rest hits the character, with different values for different types of attacks. If the coverage roll fails, the armour doesn't absorb any damage at all. It looks like a pretty realistic and comprehensive approach, but a lot more figuring and record keeping. I think Conan uses the same system, but I haven't tried it yet.
Shields usually get kinda short-shrift in armour-as-DR systems, but they're handled pretty well in OGL Ancients - defender makes an active roll to block with a shield. Simulates real weapon-and-shield combat nicely, where the shield blocks far more blows than the 10% or so the D&D AC system allows.