It says 10% but I dont think it takes into account the Def buff you get for foes near you. I have no idea what the numbers are, something like 10% for the first foe, and 5% for every one there after??
AFAIK Tough costs almost twice as much endurance as Invincibility.
With invincibility, the first enemy counts as 4, so you actually get 10 defence from 2 enemies. 6 would give you 15% defence. Invincibility works against most damage types, unlike tough and so helps to cover your weaknesses too, rather than just strengthening your strength.
@Unthing ... Mostly on Union.
From my experience I can say that invincibility is a really useful power. Even with only 2 or 3 enemies i have the feel, that they hit me less often.
Also the accuracy bonus gives me the little boost I need to hit nearly everything. With Invincibility my hit chance is at 90% I feel.
This is my actual build:
Stun is going to be kicked out with next respec, seems to be a bit of a waste, though it can be useful for stunning AVs or Elite-Bosses.
My slot assign levels are not exact as these shown I forgot to put in the 3rd dmg in EP earlier f.i.
Respecced at lvl 24 so Stamina is intended to be at 22 
Did it without Stamina till 24 but it nearly drove me crazy
Now I have nearly no end useage with all 3 toggles running. (assumeing there are no end-suckers -.-)
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Exported from Ver: 1.7.5.0 of the CoH_CoV Character Builder - (http://home.comcast.net/~SherkSilver/index.html)
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Name: Harvester of Souls
Level: 32
Archetype: Brute
Primary: Energy Melee
Secondary: Invulnerability
---------------------------------------------
01) --> Energy Punch==> Acc(1) Dmg(3) Dmg(5) Dmg(27) EndRdx(29)
01) --> Resist Physical Damage==> DmgRes(1)
02) --> Bone Smasher==> Acc(2) Dmg(3) Dmg(5) Dmg(11) EndRdx(11) EndRdx(25)
04) --> Temp Invulnerability==> DmgRes(4) DmgRes(31) EndRdx(31)
06) --> Air Superiority==> Acc(6) Dmg(7) Dmg(7) Dmg(9) EndRdx(29)
08) --> Dull Pain==> Heal(8) Heal(9) Heal(13) Rechg(13) Rechg(15) Rechg(15)
10) --> Swift==> Run(10)
12) --> Resist Elements==> DmgRes(12)
14) --> Fly==> Fly(14)
16) --> Unyielding==> EndRdx(16) EndRdx(17) EndRdx(17)
18) --> Total Focus==> Acc(18) Dmg(19) Dmg(19) Dmg(21) EndRdx(21) EndRdx(25)
20) --> Health==> Heal(20)
22) --> Stamina==> EndMod(22) EndMod(23) EndMod(23)
24) --> Resist Energies==> DmgRes(24)
26) --> Invincibility==> EndRdx(26) EndRdx(27) EndRdx(31)
28) --> Taunt==> Taunt(28)
30) --> Stun==> Acc(30)
---------------------------------------------
01) --> Sprint==> Run(1)
01) --> Brawl==> Acc(1)
01) --> Fury==> Empty(1)
02) --> Rest==> Rechg(2)
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Invincibility also acts as a taunt aura, saving the squshys on your party, as well as preventing the MM pets stealing all your fury.
I really should do something about this signature.
Mh well the taunt aspect is (unslotted) more or less just given, if no one attacks the enemy.
I mean yes, when I jump into a group, those who didn't get taunted by Taunt will get by Invincibility, but if someone throws in an AoE the taunt is gone...
Well but didn't have a slot for tauntig for it yet...
Take it. Take it now!
Slotting: 2 End Cost, 3 Defence, possibly a Taunt as well
[ QUOTE ]
Even with only 2 or 3 enemies i have the feel, that they hit me less often.
[/ QUOTE ]
The other day i tried taking on a toxic tarantula boss 1 on 1 got killed twice before winning. Next 1 i came across had a couple of fire tarantulas aiding and abetting and i wiped the floor with all of them. It does make a difference. 
I have 3 def and 2 end reds in it, and run it with weave.
Is Invincibility worth taking as a power now? hero builder seems to indicate that even 3-slotted with SO's, its only going to give about 10% defence against (say) 6 mobs.
The same 3 slots in tough would give me an extra 20% ish on redistance. Which would seem a better use of the slots.
Any recommendations?