Their shield increase skill is the only thing I like about their tank spec. The buff only last around 12 seconds so I think it is a headache to manage, becasue you lose those 4 stacks if you are not paying attention. Also they do not have the crowd tanking abilities a Guardian/Warrrior have. I can draw threat with smash, guardian slash and my two taunt powers. With smash and guardian slash alone I can control a crowd and do not even need to use my aoe taunt sometimes. Powertechs/Vanguards are still better at surviving though IMO.
Powertech are far and away better at aoe tanking than guardian/jugg, it's not even in the same ballpark.