I have the first 4 volumes of the manga + have seen the anime (movie) dubbed in English. You know, X/1999 doesn't seem to have an ending, and it may never have a true ending. The manga is halted inexplicably at volume 18, leaving suspense that kills a lot of fans. There are 2 major reasons for this:
1.) CLAMP wants the series to end one way, their publishers want it to end another way. What they could've done is publish the final volume with CLAMP's ending, then in that same volume, publish the publishers' alternate ending. Which may not happen anytime soon because:
2.) In 2003 (I think), there was an earthquake in Hanshin, and it resembled how X/1999 portrayed doomsday (earthquakes in Tokyo n'such; that's a MAJOR plot point). So thus, as much as CLAMP would like to continue, they fear that further publishing would cause great controversy.
Basically, it's on what seems to be a semi-permanent hiatus. (It appears the DN Angel manga is on hiatus as well due to it lacking an ending) I just hope the creators of both mangas formulate an ending ASAP.