I think you have to have a specialized army to use them at full potential, combined with a little experience at predicting enemy movement. But, that could be said of almost any unit.
This is my theory on their use. It is more friendly than tournament.
The Friendly Chaos Dreadnought:
You can load them up to the tits for whatever role you want them to play, but it will cost you. I don't have the codex handy but I think the MM, PC, extra CC, and ML are all close to eachother pointswise. I don't like the ML as it has the versatility at the cost of power. Dread MAKE situations, they don't react to them. MM or PC are deadly, but to you as well, CC depends on how you want to deploy.
A nice trick with the Rage rule is that it is ignored the turn the dread comes in from reserves. This will give you 1 shot 6" from your board edge at whatever angle you walk him in on, to the target of your choice. After that, it's anyone's guess what mayhem will ensue. But you did have all movement phase to get your units as far from him as possible or into CC during assault. Better yet, if you have rushed as much of your army across the board as possible when he arrives, the closest enemies will likely be tender scouts or deeptrikers. Heh or your other dread- who can say.
This way, you don't have to worry as much during deployment about clustering your guys away from him. A CC dread is simple and effective, but other units are probably better compared to a metal melee thing haphazardly running across the board with a TL bolter.
There is just something Khorny about a headlong rush at an enemy objective with mass KB rhino's and DP, while enemy units try to stand or capture objectives away from you, they move closer to where Defilers and dreadnoughts rumble into action, belching noxious exhaust from the tree-line, launching ordnance on the scattered formations and wading through those that thought they could outflank death itself. Sorry, imagination runs wild
Can you use them effectively? Go for it. Try your best and learn what the rest of the army can do to help them out. A lot of the "bad" unit choices are ones that seem to be made for randomness. Spawn mindless - ignored 1st turn out of reserve. Dread -same. Possessed can get outflank if held in reserve (with a lord if in a Dedicated transport). Defilers are big and scary and will be targeted first- hide them in reserve and hope they will have juicy targets in LOS unobstructed when they walk on. Randomness screws over reliability.
If you aren't willing to take a risk though don't use them...cough