We played a huge game, 12000 points per side between orcs, uruks and goblins vs elves, wood elves and Rohan. That was about 300 good models vs about 600 evil. It lasted about 3 hours, and felt great as we played it. Everything ran smoothly. there is some discussion about the death howl ability of the wraith king, we are trying it where it is used in the morale phase instead of the missile phase. This allows everything a chance to fight, a general principle in AoA, though it may not be entirely appropriate in LotR.
The elves are NASTY in this system. They broke block after block of orcs and uruks. The orcs hurt quite a few units of elves, but not nearly as many as we would have hoped. Essentially, we pushed into the elf and rohan army until they almost snapped, but ended up breaking completely just before they did.
If you put your nazgul on fell beasts, be certain that you protect them from eagles, by using several extra fell beasts. We are debating how good the beasts should be, they may need a few more attacks to make them as brutals as they were in the movie. We lost both wraiths in our battle, though they both did good things in the process. I'm debating on whether to allow their morale penalties to stack up or not.
The extremely powerful heroes worked out just fine. They were hard to kill, but not impossible.
We got pictures, I'm going to put them up as brief battle report this week.