Well, my best experience was the friendliness of Virgin Atlantic employees. On my first Virgin Flight this February, I saw anothe VS 747 and took pictures of it. It was really one of the happiest moments of my life. But then, after deplaning and sitting in the hotel with the freshly developped (1-hour-photo) pictures, I realized I had no clue which plane it was, and where it was flying to. It kept nagging me for quite a while.
Anyway, after I returned, while brosing the web, I found the home page of a VS 747 First Officer, which happened to have forums dedicated to cars and planes. I asked in those forums whether he had by any chance heard anything of the mid-air-encounter, and unfortunately, he hadn't. Out of gratitude that he actually bothered to reply at all, I sent him one of the scanned images as JPG.
I also contacted Virgin directly through their website, asking about what plane it could have been. They sent me a reply covering their 747 classics in old livery.
And finally, I also entered into correspondance with an airliners.net forum member working for VS who read my trip report.
Now suddenly, I got infromation from all sides: The forum member (JDB) sent me information on all North America-bound flights of that day with their departure times. Thereby I narrowed it down to one or two possibilities (with a clear tendency towards VS17, the eventually correct flight). The First officer I had contacted earlier replied telling me that the aircraft had P&W engines, and thereby he narrowed it down to four planes, only two of which flew across the Atlantic on that day. And finally, as great finale, the captain of the flight that I had been in contacted me via e-mail, telling me all details about the encounter. Later he even sent me some of his own photos. He had met the First Officer, and they had talked about my odd quest for information.
Well - this kind of dedication and helpfulness truely impressed me, and I simply cannot imiagine this happening in a huge airline with 100s of planes - there the pilots would probably rarely meet each other.
Even the relatively modest help I received from VS via their homepage was more than I had expected, and they had evidently invested some effort.
This was my most positive airline experience so far.