There are many, many threads on this topic. The short of it is that elite upgrades on CO can be scarce to non-existent, even for Platinums and especially on transcons. People also complain about insufficient reward availability, but I've never had a problem redeeming miles for my desired itineraries. IMO, CO has a better international business class product and has the best lounges of any US airline. In addition, CO is the most responsive and customer-oriented US airline, as is evidenced, in part, by the participation of CO Insider in this forum. Also, CO does not use offshore call centers. This is a major advantage, from my perspective.