I have no idea what the experience is like for the poors on United. If you are in their top tiers of status (Platinum, 1K or GS), they are awesome.
I travel UA all the time (I'm not going to humble brag, but I travel them a lot from IAH).
The good: They are generally on-time, they almost over communicate when there are delays, their inflight WiFi is reliable/reasonably fast, their technology and app is first rate. Generous with upgrades for 1Ks and GS. Miles are easy to accumulate and easy to claim. The pretzel bread hamburger on the select menu is coach is pretty damn good.
Note so good: business class food is "meh" (but snack boxes are a good value), United Clubs are too crowded.
My flight to Newark on Monday had an ATC delay and they kind of messed up making everyone de-plane then reboarding 20 mins later. More amusing than anything else. Newark to Chicago Tuesday was delayed due to weather (it happens)....they gave everyone in coach a free snack box of their choice.