I just bought a new Cannondale Prophet with the Lefty fork. One of the sales pitches that was given to me was that with the lefty fork, while riding at a slant, the tire remains true to the upright position. What that means is, with a conventional fork, the uphill side of the fork has to compress more than the downhill side, causing your tire to lean downhill, slightly.
I really find that hard to believe due to the modern engineering of new forks and the materials that companies use to make new forks.
My main reason for getting the Cannondale with the Lefty fork was price. The whole bike, with XT/XTR components was cheaper than some of the others I was looking at with LX/XT or LX.
Other than looks, I think it performs like any other fork. It did take me some time to get used to riding without one side (mental), but I really like my new Cannondale and would recommend both the bike and fork to anyone looking for a new mountain bike.
Mark