First of all, let me say that Joss Whedon is one of my heroes, so I may be a little bit biased.
Doctor Horrible's Sing-Along Blog definitely lived up to all of the hype surrounding it. It was funny and charming, plus what's not to love about an evil super villain who sings all the time? But what I loved most about this was that, in the span of three episodes that were all less than 20 minutes in length, Joss and co. have managed to create characters that are extremely multi-dimensional. You know what motivates each of three main characters, and you get to see how their character develops throughout the show.
The acting was all phenominal - how could it not be with Nathan Fillion, Neil Patrick Harris and Felicia Day? Fillion was hilarious as Captain Hammer, and it was amusing to see him playing a role that pretty much makes fun of all super heroes. A huge contrast to Captain Mal, which was part of the enjoyment of it all. As far as Harris, well, he made Dr. Horrible completely lovable, and made him seem completely human - which seems to be a flaw with a lot of supervillians (the ability to not seem human, that is).
And Joss did it again. Threw something at me that I was completely 100% not expecting, leaving me somewhat shell-shocked. I should've known this would happen, but alas. I never seem to learn.
Overall, it was fabulous, and I positively cannot wait until it is released on DVD.