I think they're early...
eBook readers have been around for years, but until e-Ink displays arrived, battery life was a real problem.
Now, however, the problem is in price - e-Ink is verrrry expensive - I understand the display is a substantial part of the cost ($100?) of manufacturing these things.
On the other hand - I've just bought my Sony PRS-500 for $200 on eBay - new. I have a decent-sized library of eBooks that will keep me company when I travel (which is often), and perhaps I'll buy some at some point - imagine - read a review, and own the book 5 minutes later.
In any case, eBook readers may do for the dead tree industry what digital music players did for the CD business - but not yet. There's a Dutch startup (iRex) that makes a great eBook reader that lets you annotate (on a touch screen) as you go. The idea is to replace 20kg backpacks for schoolgoing children. The problem at the moment is that not all school books are available digitally, and it's a rather hefty investment at €600 (although potentially cheaper than all the dead tree editions - depending on the eBook cost).
Amazon's mistake is to overhype an overpriced product. Sony has already tried, but I think it's flopping with everyone except niche markets and early adopters.
Once the price of the readers comes down, it could really take off... Until then, the market is getting crowded