My experience is that they are not quite there yet, and are still pretty overpriced. I have an iRex Iliad, which I am very much in love with. The screen is just amazing. So easy to read, even midday outside in the screaming sunshine. I use it a lot to read both regular books and technical papers, hell sometimes even long web pages or html-based documentation / tutorials.
However with good tech books I find that I read very differently to more normal "serial" material. I often skip to read or re-read parts of chapters that could be a long way back or a long way ahead of my current position. I often also use them as reference. This is where most of the reading software in these devices falls down. Bookmarking and search is generally very weak, and this impedes the way that I like to "read" tech books.
This is improving, and I like the Iliad as it is a pretty regular linux device with an X server, so better custom reading software is being compiled and released by the community all the time. Sadly, it's also the most expensive by far, as it has a much larger screen and a wacom pen tablet built in.
Unless you want to treat yourself with a new toy to play with -- because the technology is awesome, in the very real sense of the word --then my advice is to wait for the next generation, as there are still some issues to be ironed out in the current gen of these new devices.
* Edit *
Additionally, whilst the screens look very impressive, most e-readers have a screen update time of around the 1-2 second mark. So flipping pages to find that paragraph or table you want to look at again, is really not a very good option. :-( Hopefully in the later releases of the e-ink technology, this will improve.