The first three seasons of BtVS are truly excellent. But you're in for a surprise concerning later episodes (after the main characters graduate from high school). There's graphic and gratuitous homosexuality galore, and Buffy becomes the college mattress--without paying the dire consequences for promiscuity of previous episodes (which I won't go into, in case you haven't seen those episodes). The show changed networks, and everything went to blazes.
That being said, season five also has two of that show's best episodes--"Hush" and that one about Jonathan. Depends on one's tastes, I suppose.
We started with a 3pack of vhs which had 6 episodes from season 3 pertaining to Faith and the Mayor. We fell in love with Willow and most of the other characters. Then we went out and bought season 6 on dvd. My whole family almost passed out when we saw Willow and Tara kissing. We yelled and ranted though the whole season everytime they touched each other. Actually, Willow being a witch and a lesbo makes perfect sense but it's hard to deal with while we watch it. Now we are working our way through the seasons. We are in the early part of season 3. I can't wait to watch Hush it sounds great. There are a lot of fans on imdb.com and they can't say enough about the series.