I have a Samsung N130
, which makes a great secondary device for continuing writing when out and about, but I wouldn't want it to be my primary device for writing.
It's actually got a very nice keyboard, good for a few hours without getting tired, but it's still not as comfortable as the full-sized one on my desktop.
Also while a 1024x600 screen is fine for continuing with a piece that you're writing, and could even be said to reduce the chance for distraction, a large monitor is much better for laying out your ideas and writing with reference to notes and an outline.
But for writing on the move, say on a train, a netbook works very well, and can be useful in situations where you've not got enough table space for a larger laptop (such as in seats closely-spaced with no table at all).