I bought DS an Asus for his birthday (after a Mumsnet thread). He is very happy with it, but I think we should have paid a bit extra to get a proper laptop.
Small is obviously its key benefit, but is also its main drawback. The keyboard is awkward for a touch typist, not so bad for a two-fingers person. There is basically no memory beyond what is needed for the applications - so a decent memory stick is vital.
The unit is very well equipped though, with a good range of hardware and software. The main think missing is more memory.
No I don't have one, but when all the companies have their own version out I will consider buying one. It would make life so much easier if we're on holiday or out doing something, then I can just find a cafe with wi-fi and log on for the price of a coffee.
I really regretted not having something like this last winter when we were in Australia, the cottage we were staying in had free wi-fi but we didn't have any way to connect and the nearest internet cafe was $10 an hour, which is a lot of money considering dh is a workaholic and had to log on every day for at least 30 mins. I have a laptop which I used for working from home, but it's huge as I needed the biggest screen possible, so we didn't even consider bringing it to Oz with us.
I have the 901 which is worth the upgrade imo. Better keyboard, screen and touchpad. I run Linux on it - currently the standard Linux install but will probably put ubuntu on it when i get chance.
It's not a replacement for a desktop or laptop (unless all you really do is check mail and use the web) but is great when out and about or for taking notes on at meetings etc. as it is small and unobtrusive.