Dr. Horrible's Sing-Along Blog
Dr. Horrible’s Sing-Along Blog at Amazon
I have only passing familiarity with Joss Whedon’s work in Buffy the Vampire Slayer and Firefly, but I can certainly recommend the three-part show he created during the writer’s strike and has been showing on Hulu and selling via Apples’ iTunes. Dr. Horrible’s Sing-Along Blog has some marvelous singing and acting by Doogie Howser Neil Patrick Harris, a touching performance by Felicia Day and a fun over-the-top Nathan Fillion.
Perhaps this is reflected in his other work as well, but the ending was darker than I expected, giving the episodes greater weight and pathos. I’ve bought the videos, the soundtrack, and have spent much more than that assembling a tribute costume for Halloween this year.
MEADOR.ORG 
Whedon is infamous for having very few of his on screen relationships end well. He says happy endings are boring, so typically it is reflected in his other works. He usually has a somewhat happy ending, with the characters getting the end result they ultimately wanted, but with immense personal loss, seen mostly in last couple seasons of Buffy, and Serenity, as well as Dr. Horrible.
Dr. Horrible is awesome, and if you liked it he did one musical episode of Buffy, in season 6 which was also good. I doubt it would make much sense out of context though. Highly recommend Firefly and Serenity, and keep an eye out for his new show coming next year i think, Dollhouse.
Also, apparently the DVD of Dr. Horrible when it comes out will have a commentary that is also a musical. At least that’s what I heard.