They're great. For a while, Chrome would take over keys needed to use Emacs and other terminal applications. However, internally a lot of Googlers user Chromebooks + Chromebox as their only window to their dev box. It's what we give interns now.

I'm particular to XMonad, and like being able to run a few GUI applications, but I used a Chromebook for a while and the dev experience was nice enough that I almost switched. One great thing is that the SSH client is smart enough that you don't have to use screen to handle network interruptions . The trackpad isn't as good as a MacBook, but other than that the device feels great.