Right, just a catch up.
Season 1 - Good
Season 2 - Excellent ( quite dark, rated (-1

)
Season 3 - Good
Season 4 - Bad ( went downhill for the majority of eps )
Season 5 - Excellent ( picked up on the g-men more )
Season 6 - Excellent ( again with the g-men plus good standalone eps )
Will order seasons 7, 8 and 9 sometime in April.
Seasons 1, 2 and 3 concentrate, for the most part, on standalone episodes and they pull them off very well. Season 2 is the only season that was ever rated an (-1

whilst all other seasons are (-15-). The reason for this is evident once you watch the episodes. Toombs is just outright fuxing nasty ;(
Season 3 introduced the g-men in a bigger role and as the X-Files go on you tend to crave more episodes involving or revolving around them as they certainly make entertaining and morish watching.
Season 4 returned to your 'standalone' episode but unfortunately most of them were let downs and I found I had to force myself to watch a lot of them just to keep my 'watching in order' promise. Although the fast-forward button came in handy on occasion.
Season 5 picked up on the G-men again and gave a bit more back story as did Season 6. These episodes were fantastic and highly enthralling.
One thing I noticed as I've watched all of these episodes is that a lot of people, myself included, always thought the X-Files was a 'standalone' thing up until the time when Mulder’s character left the programme. Wrong. Having watched each and every episode in order there's several back stories going on at once and even the standalone episodes continue these stories or give reference to them.
Although the series in which Mulder leaves is imminent and the X-Files changes to put Scully in the lead role, the introduction of new main characters, etc, I feel confident enough to go ahead and buy every series on DVD.
As I've said, I've just finished season six and I recommend anyone who had a passing interest in watching the X-Files when they noticed it on TV to go out and buy them on DVD.
Top stuff so far. Well worth the purchase