I enjoyed the movie very much, and after thinking about it, it's very easy to see why. My favorite part of the whole Haruhi series are the first six chronological episodes, largely because they're a self-contained story with a great plot, great pacing, great characters, and a great climax. Most of the other episodes were unremarkable. However, Disappearance was fantastic because, at its core, it's a retelling of the original Haruhi story.
Spoiler for Reasoning behind the 'retelling' claim:
To begin with, both start in the same way: we're introduced to a familiar environment, with no real indication of what the plot is going to be. The anime gave us a traditional Japanese high school, and we expected antics consistent with that. The movie brought back the characters we knew so well, causing us to believe that Haruhi was going to kick off some of her trademark craziness. In both cases, our expectations are thrown on the ground, stomped on, and replaced with something far better.
When the world gets reset in Disappearance, it takes the anime back to the very start: Kyon going to school among familiar classmates, completely unknown to the main cast. Haruhi is back to her original melancholy, and all the characters are separated from each other. Once Kyon and Haruhi get acquainted, the rest of the group comes together as well, at times almost exactly as it did in the original series. The SOS Brigade assembled, we're plunged into a new world of time shifts and supernatural entities.
And this is the point at which the movie draws its strongest parallel with the anime, only now, instead of Haruhi being the god, Yuki is. In both cases we have someone who is so fed up with the world as it is that they ultimately recreate it. However, the most striking difference is the character behind it all. The anime presents us with a seemingly normal girl who turns out to be a god. The movie, on the other hand, takes an apparent god and turns her into a normal girl. Both situations force Kyon into an altered world, and it falls squarely on his shoulders to return it to normal. Finally, after some extended internal conflict, Kyon sacrifices the opportunity to stay in the recreated world and the story concludes.
With this perspective, Disappearance is a "rebuild" of the original series more than the Evangelion movies ever were of their source material. It manages to completely rewrite the original plot while retaining the core concepts, and it doesn't sacrifice continuity with the original series to do so. In fact, it builds on the original series, taking us back to Bamboo Leaf Rhapsody, recalling Endless Eight, and developing existing characters.
And thus, I find the real reason that I love this movie while the other subsequent works fell flat: rather than a supplement, this is a retelling. It's a fresh take on a unique concept in a familiar setting, and for that reason it surpasses the original completely.