I've been a UA 1K for a few years now, and I'd say about 10% of my qualifying flights per year are on US Airways, including one last saturday! Some things to note...

1) They have these things called "choice seats" that are technically only available to their elites for free. They sell for between $5 and $25ish online to normal customers on online check-in.
2) They frequently mess up when entering your mileage plus number. If your BP doesn't stay something like "*G" or "STAR ALLIANCE GOLD" it is because they messed up. Go fix it.
3) You can't get "upgrades" but in the case of irregular ops, you get treated fairly well. For example, I was flying LAS-CLT-DTW. The LAS-CLT flight was overbooked so they called my name and put me on LAS-DTW in first class. I even got more miles since the re-book qualified for 50% EQM bonus.
4) US Airways is pretty awful. The only reason to fly them instead of UA is if you think you'd prefer a nonstop or if they're cheaper and you're paying for the ticket.
5) I have about a 50% chance of getting a "choice seat" for free if I ask the GA before boarding, if they have space
6) You can get your seat assignment on US Air's website in advance. So even though you book it on United.com, and it's a codeshare, you need to go to US's site to get your seat.

Hope that info helps! US Airways sucks pretty bad... I can't change that, but I can help LOWER your expectations for next time, so your experience APPEARS more pleasant!