Personally I passed on Sundered Souls in favor of the Violet Signet, the Mag's head ring, and later the Seventh Ring off TK trash. I felt the extra armor on those two was more beneficial than the avoidance on Sundered souls.
I thought about picking it up for the stamina when we were working on Kael, but naturally it didn't drop then.
Beside that, the Ring is one of the best Avoidance Tanking Rings ingame...
Sure the Amor on the two Karazhan Rings is nice for special Situations, but in allmost every case i would wear Ring of sundered Souls over Shermanar Great-Ring... simply because it povides a huge amount of stamina and Dodge/Defense are allways welcome...
What Satrina said. More flexibility = win. Wear the avoidance ring for Prince-like fights, and wear the others for progression raiding (assuming the EH is better on both, which I haven't done the math but I think it is).
Tank Points / EH addon is great for this, by the way.
don't pass up the ring of sundered souls.. someone above said it's ONE of the best avoidance ring's in-game... IMO it's the ONLY avoidance ring in-game (epic-quality at least). Not to mention it has more stamina than almost every other ring you will probably wear at lv 70, second only to the ring off supremus.