Just posting my experience booking some flights today. First time I've used GUCs. Trip is SFO>LHR and LHR>ATL. I had specific days I needed to travel in both directions, but time flexibility on both days.

Called over the weekend to check availability. Agent found SFO>AMS>LHR on KL metal, as well as LHR>ATL on DL metal returning - GUC space available on both. Yay! I didn't have my GUCs yet so told the agent I'd call back after getting them and booking the ticket. Requested GUCs and went to bed (yes, I should have just waited and done it quickly, but whatever).

Fast forward to today and I'm working out my travel bookings for next month. Reviewed my notes and booked the appropriate flights via DL.com, then called in to upgrade the flights.

First agent tells me no problems with the return leg on DL metal, but she doesn't see UG space on the outbound. Uh oh. I ask her to double check the correct inventory (based on reading FT reports of agents searching for incorrect inventory), and she insists that she's looking for GUC space and it's not there. At this point, I'm suspicious that I have a bad agent (you can kind of tell just from the attitude), but I'm also kicking myself because it's quite possible the inventory disappeared while I was sleeping/waiting for my GUCs to arrive. Agent processes the return GUC and tells me she will waitlist the outbound - I tell her not to worry about it as I may want to change my flights anyway. She gets a little pushy about it but whatever.

Then, I do a bit more reading on FT, particularly the KL-specific threads linked in the wiki.
Then, I finally pay to upgrade my EF subscription, and find that the KL flights I want are showing Z7. Plenty of availability!

I call back and immediately get an agent who just sounds like she knows what she's doing. I tell her I have an itinerary, I'd called previously to confirm GUC space, and I want to apply my GUC. She dives right in and starts working the ticket. As she's talking to herself while working, I can tell she's reading some sort of instructions and hear her mention the KL flight #s , which is very promising (again based on the great FT threads!). Agent puts me on hold to work with the reissue desk to button things up, and then comes back a few minutes later to confirm I did want to end in ATL and not go back to SFO - a very nice and much appreciated attention to detail.

A few more minutes on hold and a $99 luxury tax later, and I've got a confirmed itinerary, new ticket #, and layflat seats assigned in both directions. The only hiccup was I forgot to charge the luxury tax to my Amex Platinum to use up airline fee credit!