Well, wow. What to say here. The seventh and final season was a step down from the previous seasons (namely, five and six). It wasn't bad or anything, but there were a lot of story elements I could have done without. There was also a lot of rushing, as if they had to get as much done as possible before the buzzer. Seemed like the characterization suffered from this throughout. Kind of the same issues I had with Serenity, actually. Also like Serenity, it's tough to see characters you care about die in the final battle.
I may have had some issues with the final season, but it picked up hugely in the last seven episodes. The last episode, "Chosen", was pretty amazing.
I'll get it out of the way right now... this may well be the best TV show I've ever watched. What starts out as a campy "highschoolers-fighting-demons" show quickly evolves into something incredibly thoughtful, insightful, and self-aware.
Originally the series was seven years long, but for me it took a lot less time to traverse. Join me for this trip down memory lane.