cry of pain degen stack?

it doesn't does it

i.e. arcane echo cry of pain, or if 2 players use it it doesn't make 10degen does it?

No, why the hell would it do that?
All it does is overwrite the copy that was cast first.

But it will stack with other degen.

I have used Cry of Pain extensively, before, and after it was nerfed and I still like it a lot. The immediate damage went from 100 damage to 50 damage, but, when you cast a mesmer hex first, with degen, and then CoP, the enemies go down very quickly. If they are bunched together the total damage can still be over 500 HP (as opposed to over 1000 before the nerf). And, as a bonus, it is an excellent interrupt spell.

Because of the herd mentality, it is hard to get into a group as a Cryer these days. However, in certain circumstances, where there is a large group of enemies, it is as effective as nuking and is certainly much more effective than the overrated RoJ that seems to still be in demand with your average PUG.

Cry of pain actually does more damage now then it did before. -5 degen over 10 seconds is 100 damage, so it'll actually do an additional 50 damage then from before the update - 2/3rds of that damage is lost in a cry of pain spike, though.

The degen from multiple uses of CoP does not stack. This is the normal effect for any spell.

I too have used CoP extensively, both before and after the "nerf". I wouldn't consider the difficulty of getting into a group as a "cryer" to be the result of the "herd mentality", although I would agree that "herd mentality" does run rampant in GW. In the case of Mesmer it has more to do with "cryer" no longer being the focus of a group, so advertising yourself as a "cryer" is no longer of special benefit. It would be better to stress the "Interrupter" part.

And, I wouldn't consider RoJ to be "overrated" either. As someone who has used both RoJ and CoP extensively, I'd say that RoJ is more effective, overall, than CoP. Especially when RoJ is combined with things like "You Move Like A Dwarf" and/or Searing Flames, etc.

As a side note, I wouldn't consider the change in CoP to be a "nerf" of CoP. It's more of a nerf to "cryer" groups. CoP as an individual skill used by a lone Mesmer is different than it was, but still just as effective. As Killamus pointed out, CoP may even do more overall damage now, but since the degen doesn't stack, multiple uses of it aren't as effective as before.

The point of the nerf was to make Cry of Pain less effective in a group spike build. Ie, 5 people can no longer use Cry of Pain as effectively on one person. However, it still allows a single mesmer or caster to use it on somebody. That's the pain point of that change, to nerf builds revolved around the spike but still keep the skill viable, which worked very well.

RoJ is good because its armor ignoring, allows you to carry strong support, and saves alot of atts.