As long as you are careful about how and when you weave it into your rotation, it is a net dps gain. Spec imp. rend and at least 2/3 TM, then macro the stance dancing into the use of rend when in berserker stance and the use of BT/WW/Slam when in battle stance. Any time rend has fallen off and you have a free global cooldown, reapply it. A case can be made to also use a glyph of rending in the slot that would normally be allocated to execute during a single target fight; but I would hold off on that until you are consistently maintaining a high up-time.
It's not very hard to weave it in before 4-piece T10, but you get very few chances once you have 4p T10. It can be useful on fights that have a lot of times where you switch targets or run away. (popping a Rend on Sindragosa before the air or pull phases, etc)