Every time I watch this show I find more to like about it, so I wrote up a review for iTunes. There were already 544 reviews there, so it is unlikely that mine will make it to the front page. For your enjoyment or boredom, I enclose it below.
Take an evil mad scientist, a video blog on a webcam, and a musical play. Mix thoroughly. You'd expect a mess, but from Joss Whedon and friends, the result is excellence.
Neil Patrick Harris ("How I Met Your Mother", "Stark Raving Mad", and long ago "Doogie Howser, M.D.") in the title rôle reminds us that he's also an accomplished Broadway singer. With every nuance of both note and character exactly as needed, he brings out the details of who this guy is and why. Felicia Day (also seen in her own show "The Guild" as well as several other rôles including minor characters on "Buffy the Vampire Slayer" and "Monk") as the girl of his dreams matches him well, with a heavenly voice that convinces us that she is the pure innocent. Nathan Fillion ("Firefly", "Buffy", "Desperate Housewives") as his nemesis does better than hold his own in the singing department (he modestly claims that he "can carry a tune in a bucket"), and justifies his gaggle of groupies with his handsomeness and heroic acts.
I'll try to keep spoilers to a minimum, but some details will be leaked; sorry. The three main characters are each trying to improve the world in their own way but being hampered by the way their personality goes about it. Dr. Horrible knows there are problems in society and sees science and technology as the way to exert control, but wants to use it to take over the world in order to make the necessary changes. Penny volunteers at a homeless shelter and wants to open a new one, but can barely get anyone's attention to sign a petition to lobby for it. Captain Hammer is some kind of superhero, but while he often interferes with the villains' plans, he is so busy congratulating himself that he doesn't really stop any of them for long.
The plot revolves around Dr. Horrible trying to gain membership in the Evil League of Evil as well as to impress (or at least eventually actually talk to) Penny, the girl of his dreams whom he sees at the laundromat. Captain Hammer, of course, interferes with both of these goals, which leads the bad doctor to escalate the level of their conflict. The songs (not just solos, but duets and trios), in both words and music, do an excellent job of comparing and contrasting the characters' feelings as the story unfolds.
The minor characters include Simon Helberg ("Big Bang Theory", "Derek and Simon: The Show", "Studio 60 on the Sunset Strip") as Dr. Horrible's evil buddy/roommate/henchman. He subtly delivers excellent sight gags as well as the appropriate meekness of a lesser evil who knows his place.
There's no reason to pay $1.99 for one "episode". Give the $3.99 for the series and get the complete story. If you count up the singing (a dozen musical segments, at least half of them long enough to call "song"s), the whole show will cost less than the soundtrack album. Which you will also buy because the songs are awesome in their own right.