Kindle is completely useless for technical reading. I would strongly discourage anyone thinking of a Kindle or other eReader for use with textbooks.
Damn it. I ordered an iRex Digital Reader 800s for use mainly with technical documentation. I guess I can still use it for reading comics.

The main limitation with many of these devices seems to be the low resolution. Except for the Kindle DX, the resolution offered is rarely enough for reading a standard book page comfortably. Forget about diagrams or schematics.