First off, it is a MUST to see the original series in order to fully appreciate the character development occurring in Disappearance. Throughout the original series, generally all of the characters were static with very little change to who they were from the first time they are introduced. Even the two main characters, Kyon and Haruhi, are essentially the same from beginning to end regardless of watching the show in chronological order or airing order, as the only change that seemed to occur was Kyon's ability to better accept the situation.
With Disappearance, the main focus is the development of Kyon as he is sooned faced with a reality too harsh for him to accept: a life where everything is logical. The entirety of the original series was focused on the daily life of a quintet of abnormal high school students with their own unique backgrounds and reasons to become acquainted with Haruhi. So when all of the fantasy is taken away, the only "normal" person of the group is then given a choice to accept this new reality he himself had wanted, or return to how the world was supposed to be.
While at a glance this idea of alternate universe existence is a reused plot in many stories, it helps bring a real sense of indecision to what Kyon really wants and presents a real conflict for Kyon to overcome. On a side note, it is also acts as a an interesting concept to be debated during and after the movie.
Next I would like to mention the character interactions. From beginning to end, the characters all fit within the universe of Haruhi Suzumiya, but because of the normally abnormal characters suddenly becoming abnormally normal people, in comparison with their original characters, a whole new light is suddenly shown upon the personalities of these heroes and heroines. Certain traits of each character become more apparent and is then shown in the light of a "normal" person, rather than the show's abnormal individual. All of these new interactions, however, help tie back to the development of Kyon as we watch him interact with these now normal characters, and helps juxtapose their new found normal-ness with his abnormal-ness, which was not as apparent in the original series.
Finally, though this the most opinionated part, is the style. Anime itself is an art both visually and storytelling wise. Very few scenes in the movie are without reason, every scene is meant to either create tension or further the overall tone of abnormality necessary to bring out the meat of the movie: the characters. There are some nice visuals, but the main focus is never to have a flashy moment to create a wow factor, but rather enhance the experience, as visuals should.
If there is anything to complain about, it would be the run time as it is a movie that requires you to pay attention. However, every scene is necessary to create the experience and even skipping a little would dampen it and make it less enjoyable than it should be, so as long as a long run time is not a problem it is fine.
As a recommendation, I would suggest watching this in the English Dub. Normally I prefer watching anime with the original voice acting and subtitles, but Crispin Freeman does a phenomenal job voicing Kyon. Not to say Tomokazu Sugita's performance was sub-par, but Crispin gave Kyon so much life through his performance that you could really feel the emotion in his words and it never felt fake.
To break it down:
Pros:
+well-executed character development
+interesting character interaction
+directing and visuals greatly enhanced the experience
+amazing english voice acting
Cons:
-long run time
-requires full attention
