Posting solely from a PvP perspective:
2-Piece: Wither or Slow Time increases damage reduction by 2% for 3 seconds.
The two piece bonus is ok but needs to be brought in line with the other tank set bonuses. Because you do not use Wither on CD if a Shock procc-ed (otherwise you delay building the charges for Depredating Volts) the uptime for this drops under the 30% expected from the 10 sec CD for Wither. Even if we would use Wither on CD it would still make for a less uptime than the other tank bonuses, if I understand correctly, so perhaps that is something it should be looked at.
4-Piece: Wither or Slow Time reduces the cooldown of Mind Control and Mass Mind Control by 2 seconds per activation. This is helpful about 50% of the time in PvP, when you are fighting in a big group and have other folks around you to protect. As we are the preferred node guards, we do encounter a lot of 1v1 or 1v2-3 situations in which this bonus is useless. I would rather take a bonus that is useful all the time and not in select situations.
6-Piece: Dark Ward's or Kinetic Ward's duration is increased by 3 seconds and charges are increased by 3. It is very difficult to keep track of Dark Ward in PvP and I believe that most of assassins use it in PvP as soon the Cd is up to compensate for all the CCs that can hit you and the chance that it would drop. As a result, this bonus is practically useless in PvP. Perhaps we can change this 6 piece bonus to a 5 sec CD decrease for Spike and bring Spike to a 15 sec CD in PvP. Just a thought.
I currently use the old PvP set armoring for the old 2 and 4 piece bonuses (reduced Spike Cd and 5% damage boost if guarding) and the new 2 piece set (belt/bracers) . Without this reduced CD Spike is the poor cousin of Low Slash, to the point that when this set bonus will be removed or the armorings get completely outdated I will probably be forced to switch to dps. While Spike was fine on a 20 sec Cd back at lvl 50-55, this is certainly not the case anymore at 60.
As a general view on the current state of the assassin PvP tanking, I believe that we are the victim of the success of the assassin PvE tanking. Not saying this is the result of totally unremarkable set bonuses for PvP, there is more at play than that. Best way to see how well are we doing in PvP is to look around in a Warzone and see how many of us are around compared to other tank classes. If you dont see us most of the time it means we might have problems.
In closing, I am not sure how the set bonuses work for PvE, and I would certainly not want to ruin the fun of those of us who enjoy that aspect of the game. I am very happy that we have one area where we shine. Perhaps we should consider having PvP and PvE set bonuses as separate so each can address issues in their own areas.