Despite being a small machine there's plenty of power in the machine thanks to an Intel Centrino Duo T2400 - a 1.83GHz dual-core processor. You can also get the machine with faster or slower processors, but this one is on the price to performance sweet spot. It's backed up by two sticks of 512MB DDR2- 1GB is a decent amount but you'll be limited if you need to add more as both slots are taken.
The X1600 chip is listed at having 512MB but in fact this only when HyperMemory kicks in - there's actually only 256MB of dedicated memory on the chip, but it will intelligently and dynamically grab up to 512MB when it needs it. It might not have the cache of the latest desktops chips or be as fast as the Mobility Radeon X1800, but you can still play games on the X1600. I tried out Call of Duty 2, at native resolution and found it to be quite smooth and very playable though this was without anti-aliasing or anisotropic filtering, and in demanding games it will struggle with these enabled.
The faster CPU helps it outstrip the Zepto by a significant amount in SYSmark 2002 and in PC Mark 05. Though we only were able to run 3DMark 06, we know that the GeForce 7600 Go is a faster chip and indeed the Zepto outperforms the Asus here. But you can definitely play games on this machine.
It also does better than on battery life than the Zepto, lasting for two and three quarter hours - and that's on its 4-cell battery. We didn't get the 8-cell battery to test but Asus claims up to four hours.
Overall then, it's faster than the Zepto, except in games, but has superior battery life, has a larger hard disk and is quite a bit lighter, especially with the standard battery. It's also better looking. All this means it justifies the higher asking price over the admittedly bargain-tastic Zepto.
If you want a perfectly practical, attractive, highly portable machine that packs a punch for work and play then the Asus W3J should be high on your short list.