I read two Buffy the Vampire Slayer graphic novels this morning. My brother-in-law lent them to me. I was a big fan of the show (although I didn't start really watching it until the 5th season), but I haven't read any of the assorted writings.
The first, written by Joss Whedon himself, was Fray. It's set in the same universe as Buffy, but forward by a few hundred years. I had heard of the book before, but I didn't realize that it was also about a slayer. (After all, there was the whole Firefly thing...)
It was quite good. My favorite character was the plucky, talkative mutant little girl Loo (pictured). There were also some intriguing twists to the plot. That's what Joss is so very good at -- twists that you really don't expect, but that are still believable -- and I wasn't disappointed here. I won't go into detail, obviously.
I also read Tales of the Slayers. That was more of a mixed bag; not all of them were written by Joss, and you can tell. Some of those he didn't write are still very good, and I thought they were all decent. I really enjoyed one that was written in rhymes, about a slayer who is tried as a witch.