Just watched this, after about four weeks of it sitting in the DVD tub, mocking me. Overall, I liked it; I mean, all the good stuff is still there, and that's what counts. For the three of you who don't already know how the story ends, there are spoilers ahead.
As with the previous two sets, most of the extra running time comes from material inserted into existing scenes. It's mostly harmless, and occasionally brilliant; the battle scenes get some more material, all of it kick-as. There's also a nice scene on the road from Rohan to Minas Tirith with Merry and Eowyn that lends some extra depth to their later bits together.
One thing I didn't care for was the extra material added on to the Paths of the Dead scenes. In the original cut, it leaves off with Aragorn asking the ghost king, "What say you?" and you don't see him again until the black ships arrive at the Pelennor Fields. It's a great moment of tension and release, as the audience thinks, "Oh, shit, the bad guys just got backup," and then Aragorn, Gimli, and Legolas leap out of the boats and start kicking ass. When I saw it on opening night, the audience cheered. Well, here the scene with the Dead continues about five more minutes, at the end of which there's no doubt that the Dead will fight for Aragorn. Then, at the start of the second disc, there's a completely cut scene where the ghosts overtake the ships. In other words, when the time comes for that big reveal, all the tension's drawn out of it like a deflated balloon. The moment isn't as much fun when you know it's coming.
But that's my only major gripe. Otherwise, good stuff, and a worthy tribute to Tolkein's story.
Now if they'd just make a comic out of it...