Furthermore, this update has allowed custom soundtracks to be used in every backwards compatible game. However, I hear you cannot stream music from your PC to them, only with music on the harddrive. People who play Halo 2 or whatever need to confirm.

Custom soundtracks are not officially supported. You kind of have to cheat by starting up a custom playlist on the dashboard, then putting in an Xbox 1 game. The custom soundtrack will continue to play in addition to the Xbox game's music, so it's not the best "feature" in every case.

In something like Halo 2, where it doesn't really play music most of the time, I can see that being pretty cool.

Speaking of which, does anyone know why, exactly, the Guide can't be accessed while playing an original XBox game? I mean, it's able to put the damn blade over the screen, what's stopping it from letting you do messages and stuff?