I've always been a fan of the X-files. I used to watch it religiously! Concerning the lesser quality of later seasons: I think the first five seasons were the best, after that they moved the series from Canada (Vancouver, I think...) to Hollywood. The atmosphere of the X-files was always dark and gloomy, in Canada that's easier to realise than in sunny California.
The main reason, IMHO, for the lesser quality of season 6-9 is that I had the feeling they didn't know when to stop. Season 6 would be the last season, but then Duchovny and Anderson signed for season 7, so storylines had to be rewritten, etc. The main storyline of the later seasons seemed so artificial... they stretched the story, had to think of new storylines, which gave me the feeling it was more of the same. Also, the mystery was more or less gone. There were already so many answers on the whole 'mythology' storyline (e.g. Cigarette Smoking Man, aliens, Scully's abduction, etc.), my curiosity faded.
Still, I would like to have the whole DVD-series. Even when season 6-9 were not the same quality of the first five seasons, it's good quality compared to other television shows.