Looks like this one has been pretty fully discussed. But here's my experience: I've used GUCs a couple of times traveling from LAX (or SFO/SEA) to SIN with the stop in NRT. Each time when I booked the flights over the phone with Delta the segment to NRT cleared right away, but not the NRT-SIN segment. This might be Delta's standard practice not to clear GU for all segments to SIN at time of booking. They seem to do a pretty good job selling those seats for money. But ultimately I ended up being cleared all segments in Delta One. Same on a recent trip SFO to SVO (Moscow) via JFK. I was GU wait listed at time of booking but ended up clearing all segments. It might help to hold the Amex Reserve card. Hope this helps. Good luck & have fun.