I have played a few games using Mordrak and have had mixed results. For the most part he does work to scare people away from spreading out their forces, nothing scares flanking units like a bunch of ghosts popping out of some random boulder. But you need to really think ahead on what you want his unit to accomplish during a game if you want to play him otherwise he will spend all day chasing units like a dog chases passing cars.
So far they are all I have finished painting
But the real advantage I see in him is reactionary deployment. As someone who has played Eldrad many of times setting up first and then being able to move even one unit after your opponent has deployed is incredibly useful. And since Mordrak and friends can't buy any special weapons, thats for the most part the real advantage of his special rule "first to the fray". Although never underestimate mass bolter fire on a 5+ armor units caught in the open.
There is always the option of attaching independent characters to him, but I'm not to motivated to do it. The reason why is because I feel Mordrak and his buddies are largely expendable. You should put him on the table where your opponent will have to deal with him like on a objective in the no mans land or behind a building your opponent was planning on moving up and castling around. He's for the most part a unit I will leave unsupported and rather use the rest of my resources to support everything else moving up. This doesn't mean put him in first turn rapid fire zones or banshee class assault unit running distances. Just put him in a place your opponent will have to keep looking over muttering "what am i going to do about those jerks"
Throwing the occasional non instant killing wound on Mordrak is also fun, getting more ghosts after the game has started is always great. The last game I played I took a perils casting Hammerhand and was like "wait that triggers a ghost roll sweet". Just remember he is a assault unit and everybody leaves the party if he dies, so don't risk it too much. One thing I didn't catch from the leak to the final addition is they changed it to added at the end of the phase. Sorry Mesdoram :(.
"Wait what happens when he takes a wound?"
I think for now he's going to be my primary HQ choice. His surprise factor despite how well he preforms the rest of the game has always payed off.
My next two projects are going to be finishing up my buffer Librarian and a storm raven. All I have to do for the librarian is paint his staff and some detail work, but the raven will take a little more time....
He's a Force Multiplier with FORCE
I hate all the stuff they decided to put on top.
I wouldn't mind hearing what you guys think of where I put the top turret weapons instead. They are magnetized and so are the wings and tail, it allows me to keep them with the rest of my army without getting a bigger case.