I saw this when it was first released on the Internet, and couldn't wait for it to be released to buy. Being sad, I ordered it from Amazon.com and was overjoyed to have it arrive on Christmas Eve! It's great - a true Joss Whedon epic crammed into forty-odd minutes, with wonderful songs that stick in the mind, characters who grab your heart, and an ending that ... well, I won't say, just in case I spoil it for you, but it's pure Joss. As for the extras, there are mini-docs on the making of this phenomenon, some fan-created auditions to join the Evil League of Evil ... but the best is Commentary! The Musical, where for the entire length of the show you can sit back and listen to the main protagonists singing about, well, everything.
When it came out on the Internet, it crashed the server because there were so many hits, and has just won a People's Choice Award in the States, so dig out some hard-earned cash and settle back to watch a once-in-a-lifetime story (that we all hope gets a sequel) then go and buy Firefly, Serenity, Angel ...