I had a brief play with Sean's first generation model) over lunch once and was pretty impressed. And at $139 from Amazon or £111 from Amazon UK (basic wifi model, prices current as of 5th February 2011), the Kindle is really not that expensive now.
There are three concerns for me:
First and foremostly, I'm perturbed by the idea of a locked-down device with everything DRMed, supported by a proprietary "ecosystem". Since I've (personally) only really used free software for fifteen years now, this seems strange and unusual, on top of practical or ideological concerns. (The Nineteen Eighty-Four deletion fiasco happened just before I saw my first Kindle.)
Secondly, I just want one device to carry around with me, not specialised devices for all the things I want to do. The Kindle looks like a really nice device for reading books on, but though one can apparently run an ssh client on one it really isn't designed for that kind of use. (Sean argues the advantages of a single-purpose device, however, and it's not clear the kind of tablet/PDA/mobile computer I want will ever be manufactured.)
Finally, books for Kindle are rather expensive - in many cases more expensive than the printed versions! - so I mostly envisage reading academic papers on one. But I'd need to do some more testing before being convinced that the PDF rendering was good enough to make this practical, especially for diagrams.