As a SFO based CO Plat for 5 years, to echo a few things folks said:

(1) upgrades are hard to get to IAH, to EWR, forget about it unless you get a full Y, and then only if you do it way ahead of time. it used to be easy when they flew 757s with 24 seats, now its 738s with 14 seats (used to be 18 but they took out seats, now a few planes are back to 18).

(2) They don't end up covering the US by themselves well. IAH is fine for the South, but EWR is a bad place to change planes. CLE is a joke and now only has one SFO flight a day. I ended up taking NW about half of the time. This used to work, and I got upgraded about 90% of the time, then 80% and it kept slipping.

(3) which brings me to E- on CO. Only thing I can say is it is paradise compaired to NWs product. Getting food is a pleasent surprise, but it does not make up for seats with 31" pitch, and without very much padding. Since there are so many elites, any change is flights, last minute ticket, etc, is unlikely to yeild you an exit row.

(4) there is no such thing as a CO redemption, unless you get the double cost (200K for BF overseas) reward. Folks gripe about MP fliltering, but the redemptions are far better.

(5) and my pet peave was always that CO would not slot me in ahead of the ma and pa kettles on a flight. Many times I would arrive at EWR or IAH to find my flight to SFO delayed, go to the earlier flight get added to the weight list and find a bunch of ma and pa kettles getting on while I got to wait for my two hour delayed flight. Power to the Ma and Pa kettles, and for them its a good deal, not such a good deal for me, who paid 4 times as much for my ticket.

(6) CO has little redundency. While flights are far more likely to go (less cancllations, mechanicals due to new planes) if something happens you are toast. UA with its wider network (DEN, ORD, IAD, LAX) hubs has always gottem me where I need to go, although its ugly at times.

(7) and no Pacific Coverage.

The good things mentioned about CO are true.

If UA does some more cutting and tips the balance with me, I would probably not see going back to CO as the answer. Now if I lived where I had mainline service to their hubs where you could get an upgrade I would go back to them quite happily. But lets face it, their best draw is comp upgrades, and if you don't get the upgrades, then there is no draw.

Out of SFO I would probably consider AA far over them, or explore the new Delta (which I know nothing about). AAs EXP deal is nice, they have widebody service to SFO, and folks seem to get upgraded much more on it, and they give SWUs (which CO does not).