It's hard to evaluate an episode from such a late season when you don't watch the show, but even from a musical theatre standpoint, I didn't like the episode. I found it really boring--there were a LOT of ballads, which made the episode drag for me, and I found most of the lyrics heightened to the point of being ridiculous, especially in Buffy's big number at the end ("Something To Sing About," I think it's called). I don't mind that the vast majority of numbers are about character, rather than plot--especially since the revelations in those songs feature hugely in the storyline overall--but those songs didn't tell me anything about who those characters are, except that Spike and Buffy are angsty. The songwriting in Dr. Horrible, which I'll write about later, is much better in my opinion.
I don't think the episode is bad; I say a lot of good things about it in my article. I just don't see what's so great about it, and I certainly don't think it's the best musical episode ever. A lot of my dislike may simply stem from my dislike of Buffy overall. The show is too precious, takes itself too seriously, and tries too hard to be funny and weird for my taste. I know this is just a personal preference thing; again, I know a ton of people who love Buffy. It's also possible that I just don't like Joss Whedon's TV shows, since I couldn't make it through the Firefly pilot.
(Although I hear the show gets better, and if I judged all shows by pilots, I wouldn't like True Blood or Battlestar Galactica.) I like Dr. Horrible a lot, though, and as I've mentioned before, I LOVE Joss Whedon's run on Astonishing X-Men, so I'm willing to say that my taste is just weird and leave it at that.