The guy has a fantastic reputation and has releasted some pretty good tracks. I actually have a few on vinyl...
But, the only time I ever heard Tiesto play was the morning after the burn at Burning Man in 2006. He must have played all night and been drinking heavily because he was completely wasted and I heard him trainwreck at least three mixes in a row.... As a DJ, I was kind of embarassed for him.
They should have put in another DJ at that point, but it was all good. The vibe was still awesome... it was like 7am in the Desert at Burning Man, how could the vibe be anything but good???