The biggest strike against the VAIO X seems to be the heat it generates. 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.