By the way, where did you get the USB relay from? My Google-fu is weak today :-\
Ordered the relay from this place, they have quite a selection of different ones,
I plan to make a Sous Vide machine as well, but I'll be using an Arduino. It just seems a lot easier (at least for me).
For me the Arduino option seems more awkward, as as well as requiring more wiring for the probe and relay (instead of the pretty simple pluging in of a USB connector) it also needs a way of looking at and setting up the target temperature. You can certainly do a tidier solution with the Arduino and I have seen some nice touch sensative rotary potentiometers that could do a very nice interface for setting the temperature. But for me certainly I find it easier putting most of the work into software.
Also there is the nice fact that nobody has done one with a rPi yet. If I am well prepared I can be the first!