I've yet to try out custom soundtracks on the 360, although I know at least one internet person who refuses to play any game that doesn't fully support the feature. I think I was just convinced, completely by accident.
I'm playing Grand Theft Auto: Liberty City Stories on the PS2. I had the TV muted and was listening to "New Noise" by Refused on the computer (you can listen to it here if you've never heard it). About 3/4 of the way through the song, there's a soft interlude, which coincided exactly with my taking a car off a huge jump. You know the type of GTA jump I'm talking about: slow motion, multiple camera angles.
But it gets better. There's a slow build-up back into the main part of the song, which culminates in four percussive blasts. The first one hit at the instant my car hit the ground. It was amazing. I'll never be able to replicate it. But that's probably what custom soundtracks are for.