As a CO Plat in CLE, I obtain upgrades 90% of the time at the 5 day window. And, I can assure you I hardly ever purchase full Y class fares.
I think it is about time that we stop accepting as fact the statements of some posters about the lack of UG's on CO. I am running about 80% upgrades on those flights that I have not bought Y fares (which are running at 100%, not counting Kenjets). The flights I am not successful on are the CO plat specials (CLE-IAH Fri PM or IAH-CLE Sun PM or early Mon AM). Even on some of those flights I have gotten a battlefield UG (much to my surprise). So, I guess life must be good for those of us in CLE. I have not yet flown on NW so I have no idea what the UG situation is for them, but I suspect I am going to find out very soon. However I think that CO UG's are more common than most people let on. Finally, getting an UG is definitely worth it for all the reasons previously stated.