I'm actually using the Armor as DR & Class Defense Bonus variants from UA in my current campaign.
I've found the biggest problem is having to re-write or convert monsters on the fly and lower their AC and give them a Number/- DR on top of their existing DR. The players like it because it makes the monsters easier to hit (and they usually do enough damage to blow through the DR and barely notice it, but it sometimes adds a round to a monster's endurance in big fights).
The players seem to like it, since it gives them the perception of being able to stand longer in a fight. However, it does make the confusion of having DR from spells (like Stoneskin), from Natural Armor or Creature Type/Class (like a Half-Dragon or a Contemplative) and from Armor, and having to announce the type of damage to a player when I tell them how much they've taken, so they can see which DR applies to that. Players who have things in order .
I like the idea of heavy armor making you more durable against little weapons, or taking a little sting out of big hits, but if I was doing to do it all over again, I don't know if I'd use Armor as DR in D&D, since it really does add a lot of paperwork and bookkeeping for not a whole lot of added benefit. I might add it to selected monsters though, to ones with high Natural Armor but no DR from other sources, for the benefits that gives (players are happy because it's easier to hit, but the monster lasts longer in combat). Successful attack rolls make PC's have more fun, even if they aren't doing as much damage.
I would probably use it totally in Modern though, to model more how modern body armors work against bullets (reducing the damage a few points to represent the broken ribs of a stopped bullet instead of the wounds of a gun hit.). Also because Natural Armor bonuses are a lot rarer in d20 Modern for PC's too, and so is DR from spells and items, so PC's generally only get their DR from their physical suit of armor.
I've also made it crystal-clear that Armor related DR applies only to weapon attacks that require an attack roll, not to a Creeping Doom spell, Coup de Grace or anything similar. In the later cases the chinks or gaps in the armor coverage are too easily overcome.
