I've been elite on UA, AA and DL. They all have their pros and cons and are inconsistent, as is the norm for US airlines.
Oddly enough, I found the soft product on AA TPAC flights to be better than UA, particularly flew the LAX to NRT/HND a lot. UA has some great people too. But I have experienced nasty check-in agents at ORD, LAX, etc. as well.
DL generally has the better soft product, until it isn't. There are nasty people over there too. YMMV