This review of Knoppix (3.9!) is based on what you actually can do with a computer without a harddrive!
The article at flexbeta has no clear conclusion, but discuss from this point
Knoppix, a Linux based live CD, will boot a computer from the optical drive and gives a slow but usable graphical user interface. The average Windows user may balk at the idea of learning Linux. On the other hand, anyone willing to fumble around in the GUI for half an hour, and throw out the occasional Google search shouldn't have too many problems using the Knoppix interface. The most recent version, Knoppix 3.9, comes with Firefox and OpenOffice.org 2.0 Beta, so if you're already using the Windows equivalent, Knoppix is that much easier to pick up. This still leaves the question, what can be done on a computer without a hard drive?