The first 2 posts sum it up pretty well.. a pure DH army list that takes multiple IST Squads with 2 Meltas in a Rhino is more competitive than a pure GK list.
Fluff-wise they should be elite crack troops that kick arse, but sadly that's not the case due to the outdated Codex. In the new rumoured GK Codex they are supposedly
getting a buff...here's hoping.
As far as WH lists go I've never seen them used..not much point when you can field SOB's for not a whole lot more points, plus they don't earn you any Faith points so even more reason to not take them.