I bought one many years ago, for cycling and walking. At the time I researched it thoroughly, and would comment as follows;
Please note that thankfully, since buying it I have never had to use it "in action".
The Royal mail training department with whom I made contact said that their trials showed that they did work, but that individual dogs got used to them and they became ineffective. This made them useless for Post staff, who always had a problem with the dog at "No.13" or whatever. This observation has also been supported in the last 12 months by an aquaintance who works for e-on, re meter reading.
Having said that, I know that Defra farm inspectors are issued with them.
Again, through another contact, a council dog warden said that he experimented with it on his own dogs. One looked at him as if to ask what the hell he was doing. The other wee'd itself and ran off...... Probably not a fair trial as the dogs were not agitated.
I have a previous work colleague who used one on holiday in France, and it worked, and that was against a nasty dog indeed which terrifyingly was running around with a broken chain attached to it's collar!
Subsequent discussions on this forum - if you can find them, will show a pretty much a split set of opinions. Some have good experiences, some have doubts, or ineffective experiences.
As for me? I carry one when in unknown areas. If there is just a 5% chance of detering something trying to stick fangs in me (and average evidence does suggest that there is a chance that it will work), then I'll carry it - albeit as a last resort!
In your case, the problem with a specific dog, may not make it the best option.
You may like to Google "Bite Back" Dog deterent spray, which is used by one company I know of - I'm pretty sure it's legal!?
Not sure if it is available retail though or to anyone not in a uniform.
Hope this helps