I believe that US Airways and United Airlines partner their miles. Can somebody give me guidance on how to book a flight on US Air using United frequent flyer miles?
Consensus on www.flyertalk.com is that to check Star Alliance availablity, aeroplan.com and ANA's website are the only Star Alliance sites which will show SA carrier flights.
www.united.com only shows United flights. If you don't want to call to check availability, check out www.flyertalk.com (United Mileage Plus forum) to see how to book.
That is how I use some of my many many thousands of MP points.
Be prepared that there are few flights available for 25,000 roundtrip. Most are 50,000 (you can sometimes get the lower miles for one of the flights, then it would be 37,500).