As of today, only the first two acts are available (the third will be up on the 19th), but it's well worth a watch. But act now, as they will only be available to view for free until the 20th! They are also available on iTunes (for a fee).
But how good is it, you ask? The first act crashed the server in its first couple of hours and it's the #1 video on iTunes. And really, it's Joss Whedon, Neil Patrick Harris, and Nathan Fillion, could it be bad? I think not.
Seriously now, it is imperative to your health that you go, as fast as you can, and see Dr. Horrible. Really now, GO!