At slvl 16 it really looks awesome - insta restoration of 65% HP, only 6 second downtime…and this 272 HP/sec regen for 10 sec…
is it really that good as it looks on paper? 
I am still on the fence as to the second mastery for my DEE caster and I see survivability might be an issue…
Yes, it’s a really helpful skill, but there are two things you might have not considered:
It does not have 6 secs downtime since the cooldown starts after the HP-reg expires.
You need to have a two-handed melee weapon or a shield equipped to make the skill work.
Right…2h or shield…thx for drawing my attention to it…So nightblade it is and this skili save for another toon 
I find the skill sort of useless as I almost always pop a health potion long before I reach that low health. It only procs against a few bosses and usually only once. I will put 1point in it most times unless I’m doing crucible.
Even worse because if what I understand is correct it will only proc when you get hit after falling below 1/3 HP.
That means you can be OS from like 34% HP and it won’t even proc, also another hit after falling below 1/3 is too risky to try and take.
The skill is incredible.
Can’t count how many times this skill saved my ass, specially against the Mad Queen. I always pick it up when i go 2H or shield with a Soldier build.
It’s incredibly worth it if you want less or zero downtime due to character death. When you are in Ultimate and in a nasty fight, or take a juicy crit, especially on squishy characters, you will see Will proc often and it’s great. On my tank characters, its mandatory and I take it over other things immediately. Well worth it.
Having Turtle & Menhir’s Will makes you incredibly durable.
It works best with a huge HP pool. It won’t work if the crit outright takes you down in one hit. You have to be able to take that one hit, then proc. So more HP is better for this.
Very best,