I think the new allied unit rules in 6th ed are an excellent idea. It's not like you couldn't just do this sort of thing before anyway if you liked (and your opponent agreed), but having it codified really does give you a nudge towards adding a little variety to your painting. It's also well-timed as now the Dark Angels minis in the Dark Vengeance set are fine to add to my existing marine army, and there's no reason why the Chaos element couldn't work as somewhat uneasy allies of my Orks. Vive la difference and all that (unless you play 'Nids I guess).
I've read varying reviews about the Hellbrute - I think it looks superbly chaos-y.
It's all being kept until Christmas now though, because there's PLENTY on the painting list to do yet...