The iPad can be used very easily as a writing machine. The lack of a physical keyboard can be a bit annoying when writing long pieces (which may go some way to explaining the brevity of my blog posts!) but of course one can be purchased for a relatively small price.
This post on the ZD Net site outlines just how one writer managed to use the iPad as a useful writing implement. It is certainly better than lugging a typewriter around.
I use it for editing student work and for blogging and communicating with others but still have been returning to my MacBook Pro when longer pieces, like my Masters papers, are due. This may well be a habit so next time I will try using the iPad and see if it is really easier. As more schools move to using iPads and other tablets the issue of writing, not to even discuss handwriting, will become an important one that may determine the success of the device.
One alternative used by some schools precisely because of doubts about the capacity for th tablet to be used for extended periods of writing, is the net book. My biggest issue with these, despite their obvious lightness and easy portability, is their processing power and start up time. They often only come with 1GB of RAM, which clearly limits their potential to multitask as well as perform many of the jobs that a tablet can do so easily.