I've used "bear spray" on aggresive dogs, and had it work very well.

The big problem was holding my 105lb Akita with my left hand, while trying to break out and 'arm' the spray with the right hand.
<img src="/images/graemlins/crazy.gif" alt="" />

Once that was done, I had to judge the wind (I have 'maced' myself before) before I sprayed the other dog in the face at 3'.

The time I maced myself was when I attempted to break up a dog fight between my Akita and a neighbor's Doberman. It worked in the sense that they stopped before too much damage was done, but I got the full effect of it too. Very unpleasant not to be able to breathe!