I think dreads are an excellent edition to almost any army. However, id recommend you never field more then 2.
Formations are also key.
One of the best is TLLC, ML, Tank hunters, venerable, extra armour.
It has the edge over the preditor because it can move and shoot. Also, iits about the only 10 strength weapon space marines have, besides the really short range of a demolisher.
You have a pretty good chance of hitting with the Lacannon as well, and a fairly good chance of scoring another hit with the ML.
Again though, the big advantage will be on turn one, Example: if you use a preditor, and you start out of LOS to the enemy, youll get 1 S9 shot. While the dreadnought gets 1 S10 shot, and 1 S9 shot.
Again with his manouverably the tank can match up the field and take table quarters, or what ever else thats needed to be captured.
Another patern of dread is the more standard, Assault cannon, DCCW, extra armour, drop pod.
When put into a drop pod, this can be a leathal combo. Also, when im drop podding I like to take it with a heavy flamer. However this breaks one of the flamer tactical rules, but i find that i can usually make up the 10 points of the flamer, on the turn the pod lands, even though it cant move 12"s.
The AC with storm bolter or flamer is pretty leathal to what ever shoots, be it space marines to DE.
Many opponents also lose there kool, knowing a pod of melta death is about to rain down upon them, and the sheer pyskological factor is probably worth it.
More of a one trick pony idea here, but, if you pod near a wraithlord, but use the pod to keep you out of LOS of the lord, (and charge range) If you give the dreadnought furious charge with venerable, it can be a pretty great anti wraithlord machine.
Anyway, the dreadnought is a huge threat to the eldar army, so they do need to invest a way to kill you, often they wont mind trading a wound or 2 off there wraithlord. With a big of extra shooting, say from a lascannon in a tactical squad somewhere, you can get the lord down to 2 wounds. Now with furious charge, youll strike first, with 3 attacks, and pop the wraithlord before he even knows whats going on.
The same thing works against Chaos lords, without a rune. (however your dreadnought is gonna die) But, you instant death the Chaos lord, which is pretty fun.
Again, these are probably only going to work once.