I liked Buffy a lot. When it first came out I found myself groaning inside a bit, thinking it was aimed at the hip teen crowd. (That was a large portion of the audience to be fair, although it's possibly aimed even more at the 'not so hip' teen crowd..) However, when I gave it a chance, I found it to be a very interesting show. The character are all rather likeable. There is a lot of humour in the show, some of which was hilarious, but it also has that dark edge that you would expect from a horror show, and there were some episodes (Hush stands out in this regards) which are actually rather chilling. And it's not afraid to be very serious and poignant at times.
There is also a lot of complexity in the characters. They will play with certain stereo-types ('archetypes', might be a better word) , then do something to subvert them. Without spoiling too much, Giles, the stereotypical foppish English librarian* would be a good example of that. Cordelia the spoilt arrogant pretty cheerleader is another good example of an archetype that is subverted.
The secondary characters are also almost as important (if not as important) to the show as Buffy herself. They are very much the heart of the show.
I don't like everything about it. I sometimes felt they were being a bit overly raunchy and shocking for the sake of it in later series (they'd moved to a different network with more relax rules I think, so they decided to be 'more adult') , but I know many would disagree. (There isn't a lot of that, to be fair, even there, but I guess those little bits stood out, for that reason.)
Also, one of the romantic threads which I disliked, I know is largely loved by many, so obviously these things are a matter of opinion. And it did pay off later...
I think the positives of the show far outweigh the negatives, however, and I regard it as one of the best programmes that has been created. Angel isn't bad, either, although I thought it got a bit silly with the Cordelia/Connor arc, but again with the opinions.
Strangely, I never did buy the box-set of this show. Maybe I should rectify that, at some point.
*the type of character- not always a librarian to be fair - who is seen as quintessentially English by the Americans, who we actually don't see much of here in real world England, although I'm sure there are plenty of his type about.