I've only gotten one and it is depressingly underwhelming. Some sort of rather small PBAoE that seems to do very little damage for the amount of energy it costs and the recharge it has.
Most "ultimate' attacks really suck. The only two good ones I've seen are Implosion Engine and Unleashed Rage (and the latter requires you to have an 8-stack of Enraged! to make it good)
Fury of the Dragon or whatever it's called in the martial arts sets is pretty decent if you have lots of focus stacks, I believe. Much like unleashed rage in that regard.
Energy storm in all the elemental sets is pretty terrible from everything I've heard, while Planar Fracture from any of the magic/supernatural sets is kinda meh but still passable. I have it on one character for concept, but I don't use it that often.
Basically your Ultimate attack isn't normally affected by your stats.
This means that, the base amount of damage it's doing is relatively huge compared to the base damage of a power like Havoc Stomp.
yeah unleashed rage is cool.
But for one toon, because it fit the concept was the one for the super natural set, a Damage over time. It does so-so damage but it fit perfectly for the toon I have it on.