I have one!
I have found it to be as useful as a hand brake on a canoe...
The problem I had with it is that
1) It is pretty flimsy
2) You the scale on mine is marked in inches NOT degrees
3) The bit that you rub up against the tyre is difficult to align properly - so you can easily get a false reading. (It would be better if it was modified to bridge across the rim rather than the tyre - but I couldn't be bothered with it)
See:-
http://www.peachparts.com/shopforum/showthread.php?t=284337
For how I ended up doing it!