First things first - there are many vendors other than ASUS (who produce the EEE PC range) who produce comparable machines - I don't have any commercial or personal reason for advocating ASUS devices.
Last weekend I a friend of mine visited most of the electrical stores in and around Reading to figure out which subnotebook best suited their needs - I tagged along as I'm considering buying one for my Sister's Christmas present.
We both did considerable online research and spoke to our friends and trusted advisors to get a feel for which devices to shortlist. We ended up looking at every subnotebook we could lay our hands on while being armed with their technical details to save time in the stores. We both concluded that the EEE PC range were appealing as they had an integrated 1.3Mb webcam, were relatively easy to gain access to for a memory upgrade (if required at a later date) and had good screens.
Picking which machine was quite a complex affair as even having cut down the scope to an ASUS EEE PC there were so many models to choose from.
I can't vouch for the accuracy of the information (though the items I checked were accurate) but I found the Wikipedia EEE PC article very useful indeed - particularly the section a page or two down titled "Specifications".
I think subnotebooks like the EEE PC make sense if your priorities are for a small light weight machine that's inexpensive and has an excellent battery life - if you aren't so interested in getting such a small machine then I think you'd be better off buying a low end full sized laptop as it is likely to be significantly more powerful in terms of CPU and it's ability to run modern operating systems such as Windows Vista - you'll also benefit from a larger screen and keyboard too.
Subnotebooks are fashionable at the moment and I think they have their place though unless you spend considerably more than the typical machines then they are no where near as powerful as "a proper laptop". If you have the money and the inclination you can buy a machine that is powerful AND light weight though you are likely to be looking at well over a thousand English pounds per machine - some of the really fancy ones are over two thousand pounds!