I've found it to be pretty reliable at CO, although I can't recall if I get gate changes. I know that I do at AA.
But the one big advantage of CO vs. AA is that CO you just set your preference for ALL trip alerts, on AA.com you have to sign up for each individual flight you want (though it usually only takes a few clicks).

And be glad we're not flying US, last I checked (May) there was NO trip alert functionality. YX doesn't have it either. Had to call the stupid phone line!