Well, according to the US website, you can check in at the United First counter, earn the elite status mileage bonus, board early (they actually include Economy Plus boarding, whatever that is
), are exempt from blackout days (but UA
doesn't have them, do they?), reserve economy plus seats, and buy upgrades at a discounted rate (I don't know what the rate is).
I'm flying US to LGA
at the end of december, so I'll be interested to see what it's like on US as a UA