I saw a nice princess cut diamond ring at my local jewelry store not long ago and it was beautiful. It was very sparkly. I think the jeweler referred to it as having a lot of "fire." He said the princess cut could make a diamond of slightly lesser quality look really good. I don't know about that -- maybe he was just trying to sell the ring.
The princess cut diamond at least has the advantage of being something different from the standard round, oval or pear shaped diamonds one usually sees. I really like that cut, and I’m anything but a jewelry expert.