I have been using Debian Testing on the desktop for a couple of years now, both at home, and at work. Naturally for servers I use Stable.
Recently I upgraded my home desktop to Unstable.
My impression on the difference between Testing and Unstable, based admittedly on a fairly short association with the latter, is that Unstable breaks more often, but Testing stays broke for a heck of a lot longer.
The NVidia drivers, for example, have been broken in Etch (testing) for months (Well, missing actually, but you don't get much more broken than that).
For those of you not familiar with Debian GNU/Linux and don't get all this talk of "stable" and "unstable" and stuff, please rest assured that I am in no way referring to my current sanity level... Please see the following link for an explanation: http://www.debian.org/releases/
Actually, this link might be better: http://www.debian.org/releases/ What the hell - if Freespire can do it...
For those of you unable to read, please see the following explanation: dflkujtg6 534fg faag gasf yjpopiu jlkjlkyj fdsft afst tlkklgjit sdff;lstjl y f fsdkl65.
In other news I just bought a mouse. Hopefully the cat won't eat this one.