According to my friends, I am not a true X-phile (don't ask why). Oh well.
The X-Files ranks as one of the most intelligent and brilliant shows on television. It is a cinematic show, using techniques that are more associated with movies than the tv show (like the long unedited sequence in Triangle). Admittedly it was once a better show than it is now, but most episodes are minor masterpieces. But when it peaks, like with "Redux", "Triangle", and "One Son", it peaks like no other show has before.
It is completely unclassifiable. It is a mystery, a sci-fi, and sometimes self parody, and the show has several markedly sexual overtones (the cigarette smoking man). The sexual tension is what probably has attracted the most people. The byplay between Mulder and Scully is cool and reserved, yet you wonder exactly what there really thinking.
It pays homage to old shows, and movies as well ("Duel in the Sun", "Rope"). This show doesn't always give you what you want.
Though it has gotten more Hollywood-ish, it's a testament to the creators and writers that they haven't put Scully and Mulder together (will they or won't they? Probably. It is Hollywood, after all).
This show is a thinking person's show. Sometimes it does get slow, but it always remains interesting.