I've only ever heard it pronounced as Tarrask, rhymes with 'harassed'.
I did play one con game set at 20th level, in which fighting the Tarrasque was the opening battle. It was staged very much Godzilla-style - a bunch of evil summoners conjured it in the bay next to a major city, and we had to try and stop it before it hit shore and started stomping the city. Was a pretty entertaining battle, but I've no idea how closely the DM played it to the rules - as players we were mostly trying to play catch-up on everything a 20th-level character has at their disposal.