I purchased 2 tickets to London today on Delta.com. I had already scoped out a fare $85 cheaper for the exact same itinerary - dates, flight numbers etc., same class on Orbitz. I also had a $100 credit from a previous Delta problem so I needed to use Delta.com. After purchasing the tickets I filed the claim for the fare guarantee (this was this morning around 10.30am.) I received an email response 10 minutes later saying they couldn't find the cheaper fare on Orbitz so they were rejecting my claim. I do understand that fares change frequently so I went back to Orbitz and - low and behold, there was the cheaper fare, still available. We emailed back and forth and every time they just said that they couldn't find it. I printed the screen to pdf and emailed it to them. They didn't reference it, just said they were rejecting my claim. It's now the same day and at 3.30pm I can STILL get on Orbitz and find the cheaper fare. Obviously it's just a rip off. If I hadn't had the credit I would definitely have cancelled my itinerary and bought tickets elsewhere. It's a small amount, but it's the principal of the thing, and false advertising.
I have flown to London 4 times on Delta this year (I used to like Northwest and just stayed with the route when Delta bought them out). I hear Virgin Atlantic and Continental are fairly decent.