I'm thinking somewhere in between, |2eason's and delphidb96's takes on a device.
Certainly e-ink (even Vizplex) isn't up to a full computer standard, but we're also not
talking about a full up computer, per se.
I think that enough folks would find it more than acceptable for a lot of everyday tasks. Reading things
other than books is a no-brainer, of course (e-mail, reports, legal briefs, journal articles, etc., etc.). I also think, based on what I've seen of the iLiad's 'drawing' functions, that note-taking would be acceptable to mosts, as would marking up reports or grading papers (again etc., etc.).
I don't think it's (quite) up to multiple windows and drag and drop ... I think the ghosting would be nasty, and the lag is still to great to be able to handle precise enough object manipulation, but that's my opinion. If somebody tried it and it worked well enough for enough folks, then I'd happily count my opinion proved incorrect.