In the group where I DM, we started in February 2001. There were two restarts (one due to a TPK, one to a vote of the players), so they're only 4th level. Highest level was before the TPK, with a couple PCs at 10th level.
With the vote to restart, it was mostly due a dissatisfaction with the disparity in levels due to character deaths (the highest leveled PC bringing it up, no less!) So, we changed the rules on bringing in new characters, and they all started agian at first level. Interestingly, so far there has been no need for new characters...
In the group where I'm a player, there have been so many restarted campaigns recently, that it isn't funny (or much fun...) Seems we've been suspended from RPGs the past few weeks, playing only board games when we meet. Fortunately, one player has offered to start a supers game. Hope it goes well.
FM