I have the Asus 1000H.
It's great for travelling with, or light use, but I wouldn't want it as my main machine, for a number of reasons.
1) They're lower-powered, and thus slower than larger laptops. However, for email, web browsing, word processing, and even some light gaming, you won't notice a difference. They give good performance for these light uses, and last a long time too (I regularly get 4 hours' usage).
2) If possible, try the keyboard first. There are two issues here, size and key placement. Some of the smaller netbooks (7" screens or even perhaps 9" screens) have very small keyboards that aren't too bad to type with, just not great. They also tend to be of a lower quality than higher-priced laptops, which may be an issue if you are going to spend a lot of time typing on them. Also, on many of the netbooks, some of the keys are in weird and annoying places. For example, often the right shift key is right next to the up arrow key. This means that if you're a touch-typist, you could well end up pressing the up arrow instead of the shift arrow (I do).
3) Lots of them come with Linux. This is a free, open source, operating system, replacing Windows. It's very stable, fast, and more secure than Windows without any setup. It doesn't run Windows software though (not entirely true; you can use Wine software to use Windows programs, but it may be a bit of hassle). It will come with a web browser, email client, spreadsheet and word processing programs though. If you really need some Windows software that's only available on Windows, make sure it comes with Windows.