The Tarrasque is not a monster to kill. It's a problem to solve by other means. We had to deal with the Tarrasque once, but we never actually saw him, we just know it was coming and had to migrate a whole population. We went back to find a completely obliterated country.
When you ran it, did you wait for the
players to be epic level?
We were high level (around 18th), but not epic.
Did having such an overpowered enemy
detract from anything?
No, it's just a different challenge. You can build an intriguing backstory on why he was awoken, or what are the consequences (political and economical) of his appearance.
Or did your players - on the contrary
- find it too easy?
All together, how did you make it
work.. did the Tarasque fall out of
It sort of appeared out of nowhere, but we knew it was coming, and was part of a much longer and complex campaign.
and that was that or was it
part of a long-winded, many-year game,
finally culminating in a session you
titled The Call of the Cthulu
It was in the middle of a 18 months "part of campaign". Our campaigns are all intertwined and last years. I have friends who are still playing characters, children of characters and settings we started in the year 2000.