Been on a few CX inaugural flights, and there really is nothing special about the flights themselves... Some gifts are handed out (useless and not nice ones) and the pilot might make an announcement... But that's pretty much it... On arrival in the US, they really don't do anything since you enter a secure area before immigration... the US side usually has more fanfare from city and airport officials for the inaugural flight back from the US...
To be honest, if you are booking the flight for no other reason except to experience an inaugural flight, I would really skip it.. but if you actually have a reason to go to IAD or something around that time, then sure, maybe a slight detour or change to a day later, etc might be kind of worthwhile...