The 17" laptop is too big to be a laptop you carry with you everywhere. Heck, the 15" is a little too large for that, too. But it's a great way to get a desktop machine that you can carry between home and work, etc. Or that you can readily move from the desk to the couch...

I think the 12" is much more of a "sweet spot". I'm gonna have to upgrade from my iBook to a 12" powerbook a little while after somebody gets Linux ported over to it.