It seems like every dual-Elite on the CO board (i.e. NW or DL Plat & CO Plat) reports much lower upgrade percentages on CO than on either NW or DL. Particularly out of EWR, I know that upgrades on many routes are very hard to find. Out of IAH, things improve, but I know my wife batted less than 50% when she was doing the BOS-IAH run every week as a Plat. On some routes, particularly transcons, upgrades are portrayed as basically impossible. So, if you primary concern is upgrades, then I would say one should think twice about switching to CO.

As for the rest of the program, there are a few plusses and minuses. On the bright side is that the product is pretty good, as is the service. On the minus side is that most would tell you that CO employees are not empowered to help you if things go wrong - status or no status. The joke is that everything is blamed on weather and, therefore, you are on your own.

With regards to redemptions, it is again a double-edged sword. I find availability on CO metal to be absolutely dismal. Particularly if you are trying to book more than a few weeks out, you will see very poor availability for most in-demand routes. In my experience, we are talking as little as 25% of the availability on most other carriers, including NW and DL. On the bright side, though, they have several good partners, including VS and EK. I burned over 500k miles in the past 18 months with CO, all on long-haul saver J, but only one ticket was on CO metal (booked maybe a week out). At the end of the day, as long as you don't have your heart set on a CO metal redemption, then sufficient legwork should keep you happy enough, but I would think it will be a definite step down from what you are used to with either NW or DL.

The 50% EQM thing is widely known, so no need to repeat it, but one should consider the limits of CO's relatively small network when assessing this. Currently, you are dinged on codeshares, regardless of where you buy your ticket. Also, if you purchase through a corporate TA and are expected to purchase lowest fares, then you are left choosing between unethical gaming to get your EQMs or traveling twice as much as you would now. And don't forget that it can seriously cut into your options and/or force you to pay a premium when flying partners.