Mass Loaded Vinyl vs. Foam for sound deadening

[farok]

I was talking with a friend of mine today who has had a number of conversion vans in the past, and he swears by MLV (mass loaded vinyl) for its sound deadening properties. He recommends it on all walls, ceiling, and floors. I priced it out, and it's VERY expensive, and I don't see it as that much value (not to mention I don't want the bus to weigh more than it needs to). Given I plan to put 1" foam board below the plywood subfloor, as well as closed-cell spray (and maybe some foam board) in the walls and ceiling, is there any benefit to MLV? Has anyone worked with it on more than just the engine compartment?

Chris