I watched it a few years ago, and recommended it to my baby brother right away, because he loved NPH in How I Met Your Mother, and after he watched it, he said it was "okay" (he's the stoic type), and then casually mentioned that he'd watched it again immediately after finishing the first time. So... this movie that was just "okay"... he had to be sure of it... by watching it twice in one sitting?!
Anyway, I loved it so much, I even paid for the whole soundtrack from Amazon! That's *cough cough* rare for me! A few years before that, though, I had come across a Dark Horse graphic novel anthology in the library featuring Dr. Horrible and Captain Hammer, and I had felt slightly out of the loop reading it (the characters seemed too well-established for the one-shot it was supposed to be). It was only years after that that I found out that they were otherwise established in an excellent film, and it all came together.