As a mid-level elite, my experience with UA runs hot and cold. Generally, they're pretty good. As another poster mentioned, E+ makes up for a lot of ills and, so far, I've always managed to get an E+ seat. For mid-levels, UA's handling of delays and cancellations is mediocre though, at least, we're rarely shunted to the Indian Call Center, which can be absolutely dreadful. For the most part, I like UA's FAs -- they're professional, efficient and usually pleasant (though trans-Pac in coach is, sadly, a frequent exception). I also like UA's pilots -- the availability of Channel 9, which monitors ATC/cockpit communications proves them to be concerned about passenger comfort and, of course, UA is the home of the legendary Captain Denny, who has redefined customer service from a captain's perspective. A lot of UA's pilots, however, also go the extra mile for customer service. For example, it is not unusual on a long-haul for the captain or FO to give an interesting lecture about the aircraft on Channel 9.
I can't speak to the UA experience for non-elites, except to say that, at least for mid-level and above, UA appears to try to provide a distinctly better experience than for non-members. As has been stated a number of times, UA's business model appears to target the frequent-flying business traveler, and rewards customer loyalty. If you can obtain 1K or GS status, you'll have a much better experience than I, as a 1P, receive. However, in turn, I know (from watching my non-status wife trying to deal with UA from time-to-time) that I have a better experience than 2Ps, 3Ps and non-status pax.
With all that said, I can't say that I love UA. However, I've yet to have a compelling reason to switch to another carrier.