sith juggy spec

what you think on this spec as a tank , or off tank ?

http://www.torhead.com/skill-calc#101cMhZsGMRMhzdGRZb.1

You will not be able to tank in that spec.

If you arent in Soresu form, you will not hold threat, and will rapidly be smeared over the landscape.

If you are in.Soresu Form, you need a lot of talents from the Immortal tree to make your threat viable, as your focus generation gets dropped drastically.

I don't really see any tanking in that spec aside from a little bit of extra defense. It should look more like this:

http://www.torhead.com/skill-calc#101fMG0ubrkuRZhGM.1

Or this one that I've been using:

http://www.torhead.com/skill-calc#101fMG0ubrkdMZhGM0M.1

While you can switch to Soresu form and maybe tank for heroic quests or similar, I would not recommend using your spec to ever tank or offtank for an operation or hardmode flashpoint.

You can tank in Vengance, but your spec will look more like this:
http://www.torhead.com/skill-calc#101dMG0uZhGrMMhzdMM.1

For tanking Ataru form does nothing since you need to be in Soresu. You also really want some of the skills from Immortal, mostly the 12% defence and -5% enemy accuracy which can have 100% up time.

You'll be using Intervene and Force Charge as much as you can to get as much uptime as you can with their 20% damage reduction buffs (which stack).

The downsides are that without Sweeping Fury or Revenge, Rage can be more of an issue (try to use Force Charge and Saber Throw as much as you can and don't forget Enrage) and that if a boss has a conal AoE, you'll take more damage since you can't Intervene without risking the death of your party members.