You can't book reward travel on UA or any other star partner yet... but you can call in and they would be more than happy to help you.

The best part of using US miles for UA - is the 50k domestic roundtrip in ps (premium service between JFK and SFO/LAX) in three class first. It costs 70k on united - and you get access to the international first lounges which are amazing (sushi platters, showers, open bar with premium spirits). Internationally, i generally redeem miles on another airline.