The Kogans are okay price wise for the hardware - but you should be aware that theyre running gOS - a linux distribution, so if youre not comfortable not using Windows youll need to look elsewhere. Im also not sure that the build quality on the Kogans is all that great in comparison with some of the name brands (but when youre paying $500, maybe you have to expect this). Also, not sure how good Kogans warranty support is.
There are plenty of netbook options if youre happy with a 10" screen. Asus have a lot of 10" variants (too many perhaps - it gets a bit confusing arfter a while), and the Samsung 10" models also have excellent battery life.
In my opinion, the Vaios are overpriced given where the rest of the market is at price-wise (their RRP is $999) whereas the Samsung (with nearly equivalent specs) has an RRP of $699.
The HP Minis might also be worth a look.
Basically, most of the 10" netbooks are very similar - same res screen, same processor, RAM and 160gb hard drive. Many of them run Windows XP. Some of them use solid state drives rather than standard hard drives, but this will limit you to maybe 16Gb of built in storage (depending on model), also a few (like the Kogan) run Linux rather than windows.