I've eaten at very few so I don't have much of a basis of comparison.

There's one in the Wynn in Las Vegas that has a single star -- it was delicious but I wouldn't say it was dramatically better than other fine dining restaurants at which I've eaten.

There was also a single star one in a chateau in Tours that was, truly, a memorable experience. The food was out of this world, the service impeccable, the atmosphere incomparable -- I was impressed.