The first six seasons were totally awesome. Every single episode was solid. The plot arcs were spectacularly good, and the characters were wonderfully compelling.
The seventh season was pretty good too.
The eighth season ... had a lot of good episodes in it.
Then ... well, some other stuff happened in seasons nine and ten. *vague dismissive handwaving*
Tonight is the series finale. The show should've ended two years ago. It's definitely going out with a whimper, not a bang. I think TV writers must get cocky about their success right around season 8 or so, and figure they can do no wrong, and therefore it's perfectly okay to take apart everything about a show that made it a success in the first place. That's certainly what looked like happened to SG1.