It won't reliably handle many non-English characters. I won't use it for Chinese texts especially. And anything where the illustrations are critical to full understanding of the text is also useless at this stage.

It's very weak when it comes to handling most books with code samples as a critical component, but in most such cases, the kludginess of transporting Kindle text to a machine where I might use the code sample is such that the attraction of stocking up on programming references that contain significant caches of adaptable code is not really there on a Kindle -- and most publishers now offer some simpler means to supply sample code in an accessible manner if you own a hardcopy of the book.

I actually find its main use for me is as a laptop substitute, at least in settings were I'm not looking at a lot of quantitative material, and as a pinch-hitting connection to the 'net when I might be someplace without a convenient phone jack or other connection. My book collection is already too large and I won't replace most of it with Kindled copies.

Still its connectivity is useful for following a few current papers, storing public-domain classic texts for text search and reference purposes, when I want to be able to answer some question quickly, but still want to "un-plug" for the most part from phones, e-mail and other pointless distractions.

I can also store reference documents of my own on the device in what is usually a more readable form than I could managed with most PDAs, if the text in question can be readily formatted as HTML without too big a loss of readability.