As an almost general rule for manufacturer battery life claims, you should use HALF of the claimed time
if you're taking in consideration a full display brightness and permanent WiFi usage.
This "formula" doesn't always work, unfortunately Lenovo is one of the manufacturers where you have to use a more likely
40% of the claimed manufacturer battery life than the usual 50%.
To make it short: after a couple of charges and discharges(6 cell main battery and 3 cell bay battery instead of optical drive),
I get the following battery lifes(not 100% accurate but it gives you a little hin what to expect):
1. at 100% display brightness(setting battery operation brightness level to max in the BIOS too) and with permanent WiFi usage, I get around 5 hours REAL LIFE battery life. Pretty good for a 13'' laptop.
2. at aprox. 50-60% display brightness (leaving the manufacturer preset alone) and permanent WiFi usage, I get aprox. 6 hours battery life.
3. with WWAN(HSDPA) permanent usage and at 100% display brightness, I get aprox. 4 hours battery life.
Meaning: the 3 cell bay battery is a MUST if you're using your X300 very often on the run.
I usually don't need the optical drive, just for initial install stuff. Actually the bay battery is a very good thing but I wish Lenovo would have found a more elegant way to release the optical drive from the bay and to hold the bay battery. The tiny screw is doing a good job of keeping the bay battery safe and it isn't difficult to unscrew it in a couple of seconds but you need a screwdriver and this is not a good solution. I also wish that Lenovo would supply some sort of soft protection pouch with the 3 bay battery, to be able to transport the optical drive in a safe way.
Anyway: as much as I hat to say this about the otherwise great X300 but battery life isn't really great without the bay battery.
Still: with the exception of the Macbook Air, actually none of the laptops with a 13'' or larger display have a battery life of REAL LIFE 3 hours using 100% display brightness and WiFi all the time. So actually the X300 battery life isn't too bad compared to the competion. Still no excuse for the need of the bay battery but as long as there is no "wonder solution" from of the major players on the laptop market, I guess Lenovo doesn't see the need for improvement in the battery life department.