As a longtime United Premier flier, my flying experiences through SFO have been reasonably tolerable over the years. I get that things might not be as smooth for regular fliers, though. Start with check-in; it's usually fast for Premiers due to dedicated lines, but can be otherwise nightmarish for regular fliers.
Boarding has been streamlined somewhat with groups 1 through 5, although group 2 is far and away the biggest line on any given flight due to the large number of Premiers here at SFO as well as many more folks who get into group 2 by virtue of having a United credit card. If you're in the later groups, good luck finding overhead bin space.
Baggage claim often takes forever, particularly for large flights, with priority tagging for Premiers working about 3/4 of the time. If you're a regular customer coming in on a large cross-country flight, count on waits of up to an hour for baggage.
United is by no means the best airline out there, but if you are a frequent flier who needs access to a hub with lots of direct flights to many domestic and international destinations, you're pretty much held captive by United here at SFO and in the Bay Area.