I had an interesting experience with the "guarantee" about a week ago. I had purchased a ticket on CO and nothing was available on the website but H fares. I bought the ticket, and then, just out of curiosity checked Expedia. They had L fares available, which of course, were quite a bit cheaper. I called about the "guarantee" and told the agent that Expedia was offering L fares on the same outbound I had booked and that I wanted to take advantage of the CO guarantee. He essentially called me a liar saying that there was no way that Expedia would have L fares available if the CO website didn't. I told him I was looking at the Expedia page and talked him through gettting there. He had to admit they were indeed showing L fares that even he didn't have access to. After a few moments of silence, he pointed that though the originating flights were the same, the connecting flights were different. (Continental's connection was worse.) So, no $100 travel voucher, but he did manage to over-ride the computer to match the Expedia fare and refunded the difference in price to my credit card. It's obviously not safe to believe that CO.com will have the best fare or the best connection on any given route.