As a huge Green Day fan, I wasn't sure if I would like the cast recording album, as it's not Billie Joe singing on it, but having listened to the sample tracks (on the US Amazon site), think its absolutely fabulous. The female voices and multi-vocal harmonies give a totally different dimension to what was/is one of the best (IF NOT THE BEST) albums released in the last 30 years, and the additional tracks (ie Favorite Son/Too Much Too Soon + the 21 CB tracks etc) just add the icing to the cake.
****UPDATE - MY IMPORT COPY CAME ON 6th MAY, AND IT'S JUST AS GOOD AS I HAD HOPED, ESPECIALLY WITH THE TRACK 22 - When it's Time, WHICH IS ACTUALLY GREEN DAY/Billie Joe SINGING A PREVIOUSLY UNRELEASED SONG *****
Although Green Day are not singing on the Cast Recording Tracks, they are playing the instruments, so many of the tracks are very similar to the original versions, but there are quite a few that sound different (in a good way), possibly because of the female vocals. It's interesting to hear the mixed together songs - Last of the American Girls/She's A Rebel for instance. Letterbomb/Favorite Son/Too Much Too Soon/Extraordinary Girl and the Green Day track 'When it's Time' would be my favourites (at the moment).
Would recommend this as a "must have item" to any Green Day collector.