I was CO gold last year, and earned Platinum for 2000.

What I Like:
*The domestic upgrade being available with no certs/points;
*Idea of new facilities like in EWR, free drinks at Pclub, etc.;
*New 777, though I have not flown it yet;
*The priority stand-by for upgrades, and possibly general standby;
*Availability of automated e-ticket check in machines.
*Ability to fly NW, HP, and code-shares like VS and earn status miles. And ability to upgrade on NW/VS codeshares, etc.

What I don't like:
*That there are no system wide like certs for intl upgrades, the wait-list, etc.;
*The capacity control procedures on domestic upgrades;
*Just read the thread about the 737 on transcon routes. Also US Airbus equipment seems more modern. E.g. empower ports on even A319 for short 1-hour flight. Why don't 737-800 have these?
*The $75 service charge not waived usually on the phone, don't 1K's get this waived.
*The Cool Travel specials. I think US runs a much better and more organized system. I always have specials I can use out of BOS with US, and BOS is not a hub;
*Numerous targetted promotions I cannot take advantage of;
*No automated upgrade system. US and DL have this.
*The web site does not display a real time reservation on the CO system. DL, on the other hand, let's you see your live itinerary the same one a DL agent would see. No syncranization on CO.
*US has a better beverage service in first with the snack basket on short flights. Cookies, chips, nuts, fruit bars, and sometimes trail mix. Beats CO short flight beverage flight hands down.