X: The Man with X-Ray Eyes is pretty good for a Roger Corman sci-fi film.
Jason X? Well, while far from being the Citizen Kane of the slasher genre, it was considered to be the best Friday 13th sequel for a while. Maybe the addition of the X was responsible for that, despite being almost cancelled out by the Jason.
X-Files the movie? Haven't seen it so can't judge. Plus as a tv-series spin-off it may be immune.
 

This is good news indeed. My critique partner is working on a book called Catch-20XX. If the X helps it achieve success, that would be a huge bonus.
 

I enjoyed the X-Men movies without fail. Though I know someone is going to ask, what about titles with XXX in it? Though one could argued for those action flicks...