Good feeling isn't it?
I try to make it look like I'm not trying that hard when I pass.
It gets better if they are on some very expensive / fast looking machine and you are not.
I was out on my hybrid for a 50 miler into the wolds last Sunday.
The route I planned just happened (not deliberate) to use some sections of an event that was being run (bikemania or something like that). Going up a quite steepish hill I approached and passed a couple of very trick racers and guys sporting all the racer gear. They didn't look too pleased with my freindly "Hiya!" as I passed them.
I was feeling quite smug as I looked behind me and seeing them still receding.
Then a few minutes later without warning a bloke dressed in blue work overalls and a backpack on a very battered mountain bike came passed me like an train - quite literally - panting and steaming. He must have passed the racers as well as there were no junctions on that section of road, so they must have been even less impressed. I thought that I would catch him up again when on the straight and level, but as I rounded a bend at the top of the hill he was disappearing into the distance. Maybe he was late for work, or might have been on a promise. Either way the bloke was a hero! Strange day.