-Does bleeding, poison, plague and any other maledictions stop the regen.
It works against bleeding, but a fight with a dagger spec will cause you to lose more hps per round than you will heal. It works against poison as well. I can't speak to plague.

-Does it work in combat or only out of combat.
Works in combat, but with sleep (and slow, holy shit) it is exponentially more awesome

-Does it have any other benefits besides the regen.
Not that I know.

-Does it work better with high con races.
High con or use the wisdom based edge

-Does it stack with other slow affects.
Oh yeah. Slow and bird edges, Moudrilar's amulet, troll pendant, it gets insane.