US:

Pro:
1. Easiest domestic upgrades for any US airline. I'm Gold (2nd tier) and have always been in F, without exception, for the last 18 months. My wife, who flies more for business, has only been denied upgrades on major business routes at peak times (very few instances). For every 20K flown domestically, US gives Golds 18K in upgrade certificates.
2. SWU for every elite level

Cons:
1. No Alliance and current partners are decreasing.
2. Concerns about the viability of the airline.

Additional Note:
Historically, US' domestic cabin service and amenities have been a step above other domestic carriers. Current cutbacks have been much deeper than with other airlines. US now has the lowest level of catering and the fewest onboard amenities of all major domestic airlines. At current levels, they probably should start painting Greyhound on their aircraft.