Sticking to one airline makes it easier to maintain elite status, whch brings all sorts of benefits, including shorter lines at airports, access to airport lounges when traveling internationally, and double miles if you get to the Premier Executive (gold) level. This means that if you fly only about three times to Europe, you get a free trip.
United has a pretty wide national network. My home airport is Baltimore (BWI), and sometimes flying is a little inconvenient. Sometimes I take the time to route myself through Chicago (ORD), and sometimes I just go to Washington, Dulles (IAD).
United is an international airline and a member of the Star Alliance, and you can use your frequent flier miles on wonderful airlines, including Lufthansa, Austrian, Thai, BMI, and Varig (amongst others).
United flight attendants are usually friendly, accommodating, and kind to passengers (in most cases).
Frequent fliers get access to an economy plus section with more leg room.
You can get more frequent flier miles by shopping at Safeway.
They are pretty generous if you get a bump -- I have often been given $400 (in vouchers) domestically, and $800 internationally. (JetBlue, for example, never gives you the opportunity to take a bump.)
They have a late afternoon flight (UA 925) from London (LHR) to Washington (IAD)
They have a late night flight (UA 924) from Washington to London so that it is possible to do an honest day's work and leave the country that evening.
They give you full credit for your frequent flier miles even if you are flying on a highly discounted ticket (unlike Singapore Airlines and British Airways).
There are some barbaric things that you have to put up with. For example, they expect you to drink wine out of plastic glasses, but I bring my own glass! The United Red Carpet clubs are crowded and have limited facilities. But if you live in the United States, you pretty much have to stick with one American carrier, and United serves a purpose for me.
Most of all, I have had many many free trips, inlcuding a trip to Hawaii (for two), several trips to Europe, a trip to Iran and back, and several trips from the East coast to the West coast, a trip (for four) to Costa Rica, and a trip to Florida (for four).