I am not sure yet. I will probably sub terminators in for the models to try them out before shelling out for them. They are very expensive points wise and you need six of them (IMO) to run in three pairs (2 inches between pairs) as a minimum to cover the most distance in a line to get the most benefit of the mobile cover.
On one hand I think they are too expensive ESPECIALLY when you start putting power mauls on.
HOWEVER, I'd like to be on the opposition to them first to see how I would cope with a wall of T5 W3 3+ armour saves coming at me, giving a 4+ cover save to a Leman Russ squadron behind them (or 3+ if you give the LRs cammo netting).
But for a 1500 point game six Bullgryns is 20 % of your force and for those points you could simply get (almost) two more Leman Russes, or a whole another 50 man guard squad to advance alongside the 50 man blob squad that would be behind the Bullgryns.
Surely you're better off investing points in the extra models (which gives more fire power in the early game turns) rather than mobile cover that is irrelevant if hit with ignore cover weapons. Also you'd only need to kill a couple of Bullgryns and it wouldn't be too difficult to then outflank the mobile cover as they advance.
I think if Ogryns and Bullgryns were 5 points cheaper each then I would get them as I like the models and the idea and have always wanted to play them, but they have never been points effective, and I think they are still points ineffective.