Up until a month ago, i would have thought they were stupid, but with a freak mtn biking accident with fracturing c4 thru c7 and having the whole section anteriorly plated, i am now wondering which are the best model out there, !( since there are a couple models now)
Belay glasses can really save you a lot of pain after you've been climbing for a few hours. They are a bit pricy though... these guys http://www.belayshades.com seem to be the best deal out there at the moment. About $100 less than CU if you can believe that.
I've been having headaches 24/7 for over a year. They never go away, and after seeing many doctors, and test, the conclusion is that the cause is vertebrae damage, from looking up for 50 years. The doc says my range of motion is better than most young people, but my neck disc are really compressed on the back side, resulting in arthritis.
I got the CU glasses after a pinched nerve in my neck put me out of business for a few months this winter. I wear them over regular prescription glasses. I can't imagine using them on a multipitch climb but they're great for single pitches and the gym.
As with poles, I'd suggest that folks who hope to keep going for a long time might consider alleviating some of the destructive stresses of our sport before the physical side-effects manifest themselves, at which point, depending on the individual, it could be too late to regain normal function.
If you are a guide and spend much of your days craning your neck while instructing top-roped students, you really ought to think seriously about belay glasses, but anyone who spends a lot of time on some type of single-pitch climbing will probably benefit long-term from not compressing their neck vertebrae for hours and hours on regular basis.