The crazy-light Sony VAIO X got the review treatment over at Laptop Mag. Their opinion is what you'd expect: it's a reeeealy thin netbook, and their test results reveal Sony wasn't kidding with those battery life claims.
The reviewers found that the thinness comes with some drawbacks, namely heat. Everything is packed so tightly inside the VAIO X that it gets way-more-than-warm-to-the-touch pretty quickly. Luckily, the 6-cell battery raises the laptop off of the user's lap and helps solve the problem. But if you're going to use the 4-cell battery, better bring a heat shield for your legs.
Speaking of batteries, Sony worked some power management voodoo on this thing. According to Laptop Mag's tests, the thing runs for 9 hours and 49 minutes, way longer than the average 6-cell running time of 6 hours and 10 minutes.