For warlock ? Some say it gives +8 total , some say only +5 as the +3 does not stack to your warlock abyss skill
Take CTA's BO for a comparison.
It is capped at +4 on a Barb, but it does still give the +sks.
It is different from charges in the sense that you can use at your convenience, like Eni's TP,
And it is also different to staff mods which are class exclusive.
Apart from that, I've seen too many people FT and ISO +8 abyss to believe it doesnt stack.