In my limited experience with UA and CO flights, I found that UA crew provided the absolutely lowest level of customer service and (surprisingly) the CO flights I took all were totally awesome.
I have flown ex-CO crew flights between BOM-EWR and v/v, plus FRA-EWR and v/v.
I choose these now UA (ex-CO) flights to/from FRA as much as possible to enjoy the service on board, which blows away LH's, even though the beer selection totally sucks on these flights.
The last time I flew on a UA (metal) flight in F, the lead F/A simply ignored me for drink service *as well as* a taking a meal order!
(Must have been the SEN FF# I had in the reservation, eh?

)