I’ve had the opportunity to watch the episodes when they originally aired, on DVD and then on DVD again the next year, etc, etc, I’ve seen 1-6 around 5 times, and 7 twice.
I think people who dislike 6-7 are wrong 
Not because they’re not allowed to have an opinion, but because many people who criticize them (and of course I’m not taking about you) seem to have forgotten the positive parts of 6-7 and the negative parts of 1-5.
I think the real problem is that BTVS got more ambitious for season 6-7 (the First Evil as a Big Bad is a perfect example of that) but it didn’t really live up to it’s potential, and that’s interpreted as failure, and that’s extended to be a failure for the season as a whole.
To re-state/clarify: 6 and 7 tried to be something on a grand scale, Buffy was extremely depressed, Willow had an addiction problem, the First Evil is going to destroy the world (for real this time) and so on. What was the worst thing that was going on in season 4? A villain, that’s it, just a demon/cyborg trying to kill people. The season aimed low and hit it’s mark, and that’s a good thing. Season 6-7 aimed higher and missed by a wide margin, but they still scored as high if not higher then any other season.
1-5 (although less for 5) were lighter, they were easily digestible, they were about a bunch of schoolmates that fought vampires and saved the world, all the while making puns and wisecracks. Of course they had their tragic moments as well.
6-7 were about a Slayer that has lived longer then she should have, a Girl who has lost everything and still has to keep fighting, there are no authority figures, there is no one to turn to other then themselves and they end up failing as much as not (they’re barely able to keep the trio under control and those guys are mostly harmless)
6 dealt with more gray-areas then any other season. It was “dark.” Really explaining what happened in 6 would take twice as long as 1,2,3,4 or 5. Why was Buffy sleeping with Spike? What exactly happened in “Seeing Red” when Spike almost raped her? Should Spike be forgiven? I’m not going to look it up, but I remember a very lengthy thread on just that issue. How many heated arguments were there about Primeval or Band Candy?
Maybe the problem is that BTVS S6 is not the same show as BTVS S2. Some people didn’t like the show that S6 was, but don’t confuse dislike with quality, 6 was not inferior to any other season on technical merits, in many ways it was far greater.
The average quality of episodes bell-curved around season 3-5 (depending on opinions) but didn’t change drastically throughout the series. S7 had it’s share of stinkers, but also some of the best of the whole run. From episode to episode (I watch about 7-9 a week as a straight run S1-7+ATS1-5) I find myself anticipating and being ambivalent towards episodes at about the same rate regardless of season (starting @ about S2.5). There are just as many pointless episodes in season 5 as any other season. Glory is a terrible Big Bad (completely failing to capture what the Mayor had), Tara and Dawn suck the life out of the screen with their whining and stuttering, but the actual episodes are par for the course.
I hear comments about how Adam was introduced to late, or Maggie was killed off to soon, and that hurts the season as a whole, but it doesn’t hurt the individual episodes, BTVS is no more about Big Bads then it is about Vampires, Glory and the Mayor are the exception, not the rule.
Ramble ramble ramble, but maybe that’s ultimately my point: the later seasons are not the same as the earlier ones, the tone definitely changed, but there is still so much going on that people can talk and debate for hours about what it all meant, and why they thought it was good or bad.
Maybe you didn’t like them, but they are part of the story, and they make the overall adventure better by being there.
As an aside: I just watched Gingerbread last night and there is a scene where Angel is consoling Buffy and says something to the affect of: “It’s not about winning: you can’t ever defeat evil, but some things are worth fighting for anyway.” This of course is what the final message of Angel season 5 is, he gives a similar speech to the Fang Gang before the final attack on the Circle of the Black Thorn some five and a half years later… pretty cool.