Dark Bow for PVP, Rune C'bow for PVM.
Dark Bow:
It's good in PVP because it's going to hit high if you spec and you're using Dragon Arrows + you won't have anything in your shield spot, so one less thing to take priority over if you die.
Use it with any other type of arrow and it's junk.
Rune C'Bow:
I can max 41 with void, potted and broad-tipped bolts which is GREAT against NPCs, but it's not constantly hitting that high, so it's not the best thing to have in PVP although it's still effective.