The keyboard is too small (for my hands) for typing out more than an e-mail or two (I'm certainly not going to write a brief on one), and the screen is too small to stare at for an extended writing and editing session. Plus, because it's based on a laptop's design, it probably needs to be set on a flat surface like a table to be of any extended use.
Really, if I need to get "real" work done, I'm throwing my laptop in a bag and taking it with me somewhere.
Given that, for what I'd use a netbook for, I'd probably just as soon spend the extra $150-$200 and get the iPad and it's better media offerings, undoubtedly more extensive application ecosystem (if the iPhone is any indication, the app store for this thing will explode) and better user experience.
But, as I said in my last post, your mileage may vary. It's all about what works for you. Netbooks don't work for me.