The movie's theme song, "Forever Love" by X Japan which fits perfectly for the last part of the movie where Kamui mourns for Fuuma's death while cradling his head. Do remember that this is the same song played during funeral of hide, the guitarist of X Japan, who died in 1998.
Broken Base: About whether the manga will (or should) ever continue or not.
Given that X/1999 had been put on hold several times until its official hiatus in 2003 due to mirroring Real Life incidents, some people find it understandable; however, Nanase Ohkawa originally mentioned that they would continue the manga when they have a magazine that is willing to publish it, yet it's already been more than a decade and CLAMP have four works in their table (Tsubasa World Chronicle, xxxHolic Rei and Drug and Drop, as well as the newer Gate 7) without news about the series. And after they only moved the schedule to put three of them on hiatus, leaving Tsubasa as the only running work, X/1999 fans have given up hope that the manga will ever return at this point.
The situation is especially unpleasant having in consideration that most fans of X/1999 were unsatisfied enough with the run of the originals Tsubasa and xxxHolic to believe that they don't deserve to receive priority for sequels before X get a freaking conclusion once for all. However, conversely, considering the mixed reception of the endings of Tsubasa and xxxHolic, you have some fans who do not want CLAMP to finish X/1999, thinking that they would only ruin it by following the same ending style.
Even although most fans prefer the manga and/or anime over the film adaptation, it has its fans due to its grittier, gorgeous animation (some think it is still better than the often Off-Model style of the anime), comprised story and emphasis in action (which eliminated the trouble of the Kudzu Plot which was quoted as one of the low points of the series) and rotund ending (especially for those who dislike Fuuma).
The Chris Carter Effect: X1999 remains on hold since 2003. Nanase Ohkawa, the lead writer of CLAMP, mentioned that they're still looking for a magazine willing to publish the remaining chapters. However, a decade had already passed and CLAMP put twomore works on hold to work on Tsubasa World Chronicle, which is a continuation of Tsubasa -RESERVoir CHRoNiCLE-, xxxHolic:Rei which is also a semi-continuation of Xxxholic and a sequel manga for Cardcaptor Sakura. Fans of X/1999 are not pleased with this and doubt that the manga will ever continue at this point. It doesn't help that several plot points have been left hanging for more than a decade such as Kamui's "true" wish and most importantly, who wins between the Dragons of Heaven and the Dragons of Earth.
The movie version of X/1999 can be this since this was released while the manga was ongoing. The world is about to end as usual but both ended killing each other and dying at the same time. The Dragons of Earth are portrayed as mass murdering assholes which includes Kusanagi, who is known to be Token Good Teammate and a Friend to All Living Things. The Dragons of Heaven are still the good guys but they are slowly decimated along with the barriers they're protecting, which resulted to entire city completely destroyed with Tokyo Tower left standing. And in the end, Kamui is forced to kill Fuuma by beheading him which leaves him the only survivor in the cast.
The manga strays into this in the last chapters as well. The Dragons of Heaven are in a losing battle where Yuzuriha suddenly disappeared, Subaru defected to the Dragons of Earth after inheriting Seishirou's job with the belief that he wanted to help Kamui realize his "true" wish and Arashi became a pawn to Hinoto's evil side who is responsible for the mishaps that the group's experiencing. It also turns out that Kamui's "true wish" is not restoring Fuuma back to his original self. It doesn't help that the manga went a decade-long hiatus with a Cliffhanger which drove readers mad. Of course, most readers want CLAMP to continue the story. But due to CLAMP's recent works which have poor endings, some care less about it with the fear that the ending is probably be more depressing than the movie.
And if you cross paths with a Seishirou/Subaru fangirl, don't mention Kamui. Similarly, saying the name "Subaru" in front of a Fuuma/Kamui fan can be as unwise as mentioning Kotori's.
So are Seishirou and Yuuto.
Ending Aversion: A lot of people otherwise interested on the series avoid reading the manga because it has no conclusion and it probably will never have.
Ensemble Dark Horse: There are a lot but two most popular are probably the energetic Sorata and the cold yet kindhearted Subaru.
Fanfic Fuel: They are probably a lot of fanfics which had a lot of What If? scenarios such as, 'what if Kamui joins the Dragons of Earth instead and Fuuma go to the other side vice-versa'
Though they are fanfics of Kamui visiting Yuuko's shop, CLAMP made a short comic of it in Clamp in Wonderland EX in which Mokona jokingly says that the price of Kamui's wish will be the fate of the world and Watanuki scolds him for making Kamui cry. Fuuma appeares later on and declares that he's the only one who can grant his wish; cue the two fighting again much to Watanuki's chagrin.
This is essentially the Gospel According to Nanase Ohkawa. The major alteration from Judeo-Christian eschatology is the dualism, with the two sides evenly matched and only a slim moral advantage on the part of the Dragons of Heaven.
Ho Yay: C'mon, it's CLAMP. The most prominent case Kamui and Fuuma is later turned Foe Yay, natch. There are other common matchups as well, including Subaru and Kamui (mind penetration and Big Brother Mentorship FTW) and Fuuma and Seishirou (mostly for the symmetry, it seems). Seishirou and Subaru are far enough out of subtext and into text that Ho Yay doesn't really apply anymore.
They Wasted a Perfectly Good Character: The presence of Nokoru Imonoyama and the former CLAMP School Detectives in the X canon would have produced some interesting storylines had CLAMP given them a bigger role than mere one-time benefactors. Nokoru's status as the head of the wealthiest family in Japan (and, perhaps more important, their relation to the Magami clan) could have believably have him involved in the conflict in the Dragons of Heaven's behalf, Suoh Takamura would have been a Badass Normal asset, and Akira Ijyuin's role as the master thief 20 Faces would have come handy during the robbed Sacred Sword arc. Instead, they become glorified cameos, and the anime even directly ignores them.
A lot of fans just love to bash Kotori for being not only an Ill GirlYamato Nadeshiko, but for daring to be emotionally and psychologically damaged after seeing the birth of the second Shinken.
Kamui was HOTLY bashed and loathed by fans for daring to cry and angst over his shitty destiny and the terrible breaking of his, Kotori and Fuuma's bonds.
Yuzuriha, as she basically never wins a battle on-screen.
The Woobie: Kakyou, Kotori, Subaru, Kamui, Yuzuriha when Satsuki decides it's time for a very literal Kick the Dog moment and kills Inuki... a good chunk of the main cast, really. Not so strange when you consider that they're Team Keep Humanity Alive. Among the fandom, Subaru is the undisputed King of Woobieness.
Kakyou Kuzuki is a Dragon of Earth. Team Kill Humanity to Save the Earth isn't entirely without Woobies, you know? Outside the movie, pretty much everyone is a Woobie or just plain Affable. Fuuma's theme is granting wishes (read: Killing people who want to die), the Sakurazukamori assassin is a position apparently passed on by death at the hands of the one the current assassin loves (Seishirou killed his mom), Satsuki was used by first a father and then corporate businessmen who want to exploit her. Kusanagi is perhaps the only one on the Dragons of Earth who takes his side (though halfheartedly) because he's a nature-lover. (Oh, and he's ready to Back Stab FUUMA to save Yuzuriha.) Nataku hardly has a past at all because he's a clone. That leaves Yuuto, who's incredibly cheerful and friendly who's pretty much in it For the Lulz, and even then he wasn't fully evil as he has a sister he misses and genuinely cares for and refuses to take advantage of Satsuki's crush on him when he could've perfectly done so.
Nataku's case is ambiguous, since he might not have a proper past since he's a clone but in the manga what's left of the person he was cloned from's memories play a very important part in his very spoileriffic and sympathetic death. You could even make a case for the anime, as Fuuma absorbs Nataku's body to heal himself after Sorata almost kills him because of his Wish and former life's memories, in a sense.
Subaru was a Woobie before X even started, having lost his twin sister at the hands of their best friend / mutual love interest, Seishiro. Yes, Ho Yay and Foe Yay in a single package.
WTH, Casting Agency?: In the Spanish dub of the movie, Aleix Estadella as Nataku and Kusanagi was considered an incredibly bizarre cast choice, especially in the second case, as he is known for playing funny bumbling guys and neither of the two is two miles in range to fit this description. He later voiced Sorata in the anime, a much more fitting character, but then some critized it for supposedly making him sound too silly.