Just finished the Unity set for my Assassin. Only a very minor (less that one pt) drop in DPS, but a huge gain in defense and vitality and a nice little dodge boost too.
Before
After
The rolls I have rerolled were +max HP/Mana and a Melee accuracy one.
The rolls I kept were:
+338 Melee Dodge
+294 Spell Defense
+384 Ranged Defense
Not sure what the max rolls are for ANY of the stats yet, and at the moment am not prepared to go on a spending spree to find out.