I only play Khador, so I cannot comment on the other models. As for the Doom Reavers, they are a solid choice. Not a tarpit unit by any means, but they will get the job done. Get the Greylord Escort too, since he keeps them around much longer.
The doom reavers are an awesome unit. I agree with getting the UA for them. Look for ways to extend their charges or use a distraction unit to keep them alive until charge range. Keep your spacing cause of berzerk.
Welcome to Khador. Please pick up your bandoleer to the right. When the man with the rifle in front of you is killed pick it up and keep running forward.
Doom reavers are awesome, only people crazy enough to run shirtless through the khadorian woods.
As for the griffon, i only know one guy who has it, but he really likes it.
I have seen the Griffin. It seems really small...
Doomreavers are great. They can put the fear into an opponent if played correctly. Even one or 2 getting into contact can do some damage. Never played with the UA. But, having more control of berserk and tough is a good thing...