I use the following house rules for armor DR:
light armor DR 1/-
medium armor DR 2/-
heavy armor DR 3/-
The enhancement bonus of armor (but not shields, etc.) is added to these figures. Also, to balance this with creature stats, every full 4 points of natural armor grant DR 1/-. DR from these and other sources stacks (even is some of the DR is of the 10/+1 type).
Armor also provides its usual AC bonus on top of that, but then I use a system of fixed hp that don't increase with level (but AC does, a little). If you use armor DR with standard hp, you should probably reduce the AC bonus.
As for playtesting, these rules worked fine for me, but my campaign has also not been very combat-heavy so far.