I thought the ep was really cool. They didn't come out and say Buffy was really in an institution and Sunnydale has been a hallucination for the past 6 years, which is a good thing. But, they make you think about it, especially with the ending.
I mean, you could look at the ending as her leaving the asylum-hallucination for the Sunnydale reality, or did she choose the Sunnydale-hallucination, but is still really in an institution, just comatose. I definitely think its the former. Obviously the latter implies that the whole series is a hallucination, but I am really basing my opinion on the one scene between Joyce and Buffy in the institution. The one where Joyce tells her she is a strong girl and Buffy says 'thank you.' I think that scene was really important. Buffy, admitted being very despondent recently, but her illusion of her mom saying that might have finally convinced her that she is strong enough to deal w/ her new life in the sunnydale reality. So, I kind of see this as Buffy's 2nd step (1st was admitting her using Spike) towards becoming somewhat 'normal again'.
I've got a few more things to say, but I'm already late for class...
-Helga