Yeah these things work just fine but agreed using them online can be tough. I just tried to book a very simple 2 segment, all UA LAX-SEA-LAX ticket last week using a $300 cert... ultimately I had to have it booked over the phone. The website kept giving an error and I know I was entering the info correctly. I've done it successfully many times but this was my first attempt using the new website.

But OP.. as someone else mentioned.. you can't have any partner flights in the itinerary even if the UA portion by itself costs more than the value of the cert. The OGG itinerary would probably have been the more useful one for you to have used the cert on....

If you don't want your cockroach letter... I'll be glad to take it