Nothing will prepare you for the moment you first touch down on the Mun with your Kerbals - alive. The sense of accomplishment is magnificent.
Its probably comparable to the crushing defeat you feel as you realize you forgot to make sure you had enough fuel to get back home.
But! You've done it! You managed to get your Kerbals to the Mun, land them safely, lift off and get into an aerobrake orbit around Kerbin!
Aaaand, you forgot to put parachutes on your command pod :|