before I go back to sierra madre for a second run at Dead money, I'd like to have this sorted by anyone having the perk and a being a melee character : does the perk give critical damage= critical x 1.5 or critical x 2.5, wich would be a HUGE boost ? is it 150% or +150 % ? big difference here...

It doesn't affect the critical multiplier, just the critical damage.
And I have no idea, +150% would be... A LOT... If crit damage is 40 then it would mean 100 in crit damage...
Seems a bit overkill.

didn't talk about multiplier, only damage discussion here. perhaps it wasn't clear enough. thx for the insight though. I did some testing today and it seems that with Better Criticals, the damage boost is still significant, but not enough for my taste.