Seems like it works on fire warlock too? Didn't know that, that's gotta be pretty nice for self sustain and heralds

Well it does provide mana and life after killing stuff and fire warlocks kill stuff so...

hex siphon provides life/mana after each kill for every kill you get, with any attack/skill. Just not for kills by your minions/merc

Good when there are a lot of crap enemies around. Bad when there is only a boss.