My favorite experience with Early Access so far would be Don't Starve. The game was already very enjoyable when I first bought it and every month up until release they added great stuff to it, and the devs were very active on the forums communicating with their fans.
Worst would be War of The Vikings. I like the game a lot, but during early access they added almost nothing to the game up until right before it came out. Also, they haven't been responsive to what the fans have asked for. I still dig the game, but as far as the early access stuff goes it was a failure.