I've watched the series countless times before, but didn't buy it on DVD for two reasons:
1. I don't buy DVDs anymore. It's Blu-ray or nothing for me.
2. I can watch the series any time I want on Netflix Instant View
Now that there's a Blu-ray release, reason #1 is out, but is buying the disc worth the $15, rather than the free Netflix option (well, free-ish, since you do have to pay for Netflix monthly, but if you're only paying so you can watch Dr. Horrible, then I can safely say it is long past time for you to buy a disc)?
In case I hadn't made it clear above, "Dr. Horrible" holds up to multiple viewings. It's incredibly well-crafted, and tells a pretty complete story in just 42 minutes. What I love the most about it is how it turns the hero-villain dynamic on its head. Harris plays Dr. Horrible, who is a villain, but is the protagonist of the movie and also the character you're more likely to root for. Meanwhile, Fillion's Captain Hammer is a traditional hero, but serves as the film's antagonist and is also kind of a douche (I'll address that more later on). Plus, you can never have enough Felicia Day in your life.
For something that was originally filmed for broadcast on the Internet, "Dr. Horrible" holds up surprisingly well to the scrutiny of 1080p Blu-ray (and I can finally say that with confidence, since I just got my new 52" LED LCD TV last week). Felicia's hair has never looked redder. MMMM. Oh, sorry, right. Review. The image isn't going to blow you away like say "The Dark Knight" or even "Serenity", but it looks very clean for such a low budget production.
The Blu-ray gets a nice DTS upgrade over the DVD, and you can definitely hear it. For something that's a musical, the audio really matters more than the video, and that's where this release shines. However, there was one issue I had with the audio, that we'll get to in a minute.
This release is loaded with special features, the highlight of which is "Commentary: The Musical". It's an entire extra soundtrack of songs, recorded as a "commentary" track that serves to skewer the film-making process and commentaries themselves. The inclusion of this is worth the purchase price alone, particularly since you can't get "Commentary" just by watching on Netflix. However, disappointingly, "Commentary" is not in 5.1 surround sound. The songs themselves still come through loud and clear, but I could barely hear the snippets of dialogue between them. So I had to keep turning up my volume to hear those and then turn it down again when the songs started up. It was a minor issue, but an issue worth noting nonetheless. Still, it's hardly a dealbreaker, especially since "Commentary" is just one of a handful of special features that also include a traditional commentary, a making-of featurette, outtakes, a behind-the-scenes clip, and fan-submitted Evil League of Evil application videos. There's also a too-brief interview segment with the members of the Evil League of Evil. I'd love to see them featured more prominently in a sequel.
The "Dr. Horrible" Blu-ray is definitely worth owning, and it's worth upgrading over the DVD just for the audio track -- though if you're like me and you waited for the Blu-ray, you'll be happy you saved yourself $10, since the two releases are very similar. It's a must-have for any Joss Whedon fan, and even those unfamiliar with Joss's work will enjoy the story, the songs and the groupies (they do the weird stuff).
Also, I wanted to throw in one last thought about "Dr. Horrible", something that didn't hit me until last night. The whole story itself relies on an unreliable narrator. The plot is told from the perspective of Dr. Horrible/Billy, who is a villain in his world, but is cast as the hero (if not a tragic hero) in this story. However, isn't it entirely possible that there's a different version of the same story to be told, where Billy/Dr. Horrible is an annoying stalker-ish suitor of Penny who is frequently rejected and turns to evil to get revenge, while Captain Hammer is actually a Superman-esque bastion of good, without the douchiness? I wanna see someone make THAT movie too.