The Haruhi franchise is one that's very respected within the anime fandom, I particularly like "The Melancholy of Haruhi Suzumiya" series since it's one of does shows that incorporates complex themes and make them work so well that makes this animes one of does rare anomalies to recreate in any form of media: It has a very interesting way of incorporating sci-fi elements into it's seeming "slice of life" storyline; it's made even more clever by having it play a background roll opposed from having it take away from the spot light the lovable characters.
The movie "The Disappearance of Haruhi Suzumiya" is quite the fascinating movie and one I recommend people to watch, particularly if you are into anime in general, unfortunately it requires you to have certain knowledge of the background behind the universe and the characters... I would recommend seeing at least a few episodes if not the two seasons of the show. The movie happens to be a weird concoction of feelings despite at its core laying a story about saving the world but also how much is the world worth to others: After watching this film I can honestly say is one of the best animated movies I have seen in a long time placing among classics like "Ghost in the Shell" and "Berserk". I know that this sounds like an incredibly bold statement but I wouldn't be doing such a comparison if I wasn't feeling so sure about my thoughts towards this movie.
The movie deserves to be watched at least once, I know that it's a shame how does who want to check it out need to catch up to the anime, but it's still worth watching since it has mystery, humor, sic-fi and likable characters.
The anime "The Disappearance of Nagato Yuki-Chan" however falls into the spin-off category by taking a bold move and making a "What If" timeline: What if there were no aliens, time travelers, robots, etc... Despite the characters being the core center of the story, is the world itself that gives this characters and story it's flavor, removing these elements it's like taking away Lightsabers and starships in Star Wars.
This anime in question is nothing more than a romantic slice of life story about Kyon and Yuki, that's really all there is, Yuki is turned into a shy/weeboo-ish girl who although cute has none of the character the movie knew how to make work, in the original anime Yuki was like that one character you don't fuck with (To me is still a mystery how they managed to make her work there). The majority of the characters are still has they were in the original anime and movie though so is not like I don't recognize their characteristics but I can tell something is missing... and that is the fantastic elements the original story incorporates so well.
The romance sticks out like a sour thumb to me, the original anime and movie has non of that... it is hinted that Kyon and Haruhi have feelings but is never addressed to that level further into the story and is never touched or referred to ever again. I don't care that "The Disapearance of Nagato Yuki-Chan" is a spin off, to me it comes across has a fan fiction with a budget, no more and no less... I can say that it is harmless: regardless of my opinion I never felt like the anime insulted my intelligence or anything like that, so you can still check it out for yourself and judge it if you want, for me however is a definitive skip.