You will not be eligible for complimentary upgrades on US.
You will not be eligible for preferred seating (e.g., exit rows) on US (choice seating is paid for everyone, including US Preferred).
You will receive actual flight miles on US metal (no 500 mile minimum).
You are eligible for priority boarding on US.

United has a Low Fare Guarantee, but it only applies to United flights purchased online (ie, codeshares and purchases made in person or over the phone don't qualify). It would work if you purchased a United flight on united.com and later found the same flight marketed on usairways.com or some other site for less. I've actually seen this happen, although the vast majority of my travel is via corporate travel agent rendering the guarantee useless.

EDIT: incidentally, as a Platinum, I'm flying 4 segments on US next week. I'll be in 2 middle seats, 1 aisle in row 24, and a window in row 28. So yeah - definitely no preferred seating privileges.