I'll add to this.

I frequently fly UA intl, both biz and first class, between the US and asia.

upper deck is all business class. there is also biz class seating downstairs.

UD pros:

- nobody from coach tries to use your bathrooms.
- nice storage bins next to the window seats.
- not many people up there, not many people wandering around. can't hear the noise from babies crying in economy class. more quiet.
- if you're bored, you can daydream about attacking anybody who tries to hijack the plane.

UD cons:

- you can't sneak into the first class bathrooms (as easily)
- overhead compartments aren't big enough to fit a full carry-on size suitcase. you store them at the back of the UD cabin, near the stairwell.
- when the pilots use one of the bathrooms (they use the UD biz class BRs), the flight attendants stand guard over the cockpit and the empty bathrooms. ie, you can't use the UD bathrooms during this time.
- you can't watch all the other passengers as they board (if you're into people watching or checking out the opposite gender).
- you have to haul your carry-on luggage up a flight of stairs.
- it takes longer to get off the plane when you land.