Depends on your circumstances.
They are very valuable if they clear at booking as another poster said. But that is few and far between IME. Depends on your routes. DTW-AMS seems to be about the best.
If you allowed to buy J (lucky me) then they are worthless to you if it is not available at booking.
If you are not allowed to buy J then waitlisting and clearing later would be of great value. But one night in DXB there were 20 DM's trying to use GUC's and only 1 made it. I was very glad I was not in that group, I had 7A already!
IMO, one of the best uses is for mileage runs - I was saving 2 for a December run but then I ended up not needing the run.
Most of mine expire or get used on short domestic trips. Even then they are hard to use. Oh, and of course, the entire program is broken.