I live in Newark, so obviously CO has been exceedingly tempting. Circumstances for the past 1.5 years have had me flying to places that CO does not (TRI) so I stuck with US.
I'm scoring 100% with upgrades as a CP on all my CLT, PHX, LAS flights from EWR. US F isn't much to write home about, but I do drink and I am tall, so that makes it worth it for me. It's definitely better than CO Y. I have friends who are CO plats who are in the < 50% upgrade range on flights out of EWR.
I couldn't handle that with the amount I travel.
Also (there's another thread about it recently), US has been cleaning it's operation up at EWR recently, and the staff there knows me. So, I stick with US.
Sometimes I think I'm crazy. I had to go to LAX last week, and I took a connection in CLT on the way there and a connection in PHX the way back instead of taking a UA or CO non-stop. I upgraded on all four segments with no issue. As a CO Plat, upgrading on an EWR-LAX transcon is practically impossible (many many of my coworkers can attest to this), and UA doesn't give free upgrades at all - so I was fine with taking connections and having a comfy seat.