If so we could block the MULEs and scans, etc.
What about other buildings?
I'm assuming it will wipe Protoss shields. That means if you have a double ghost you could throw down a nuke with one and EMP with the other right before the nuke hits.
Yes, EMP will drain the energy of buildings. It will also do, IIRC, 100 damage to shields, but not necessarily take them out completely.
I have not confirmed, but I have read that it will also reveal cloaked and burrowed units for a period of item as well.