Having used a spray collar for certain training problems, I have as the years gone by met many people who have exclaimed what a great idea. That said when they hear the price they are often put off by this 'Great Idea'!
I also met one lady who decided that she would use an anti-bark (a spray collar that reacts to barking) collar to train her two little terriers from chasing deer. I tried to explain to her that they would in fact still chase deer, but instead they would do it quietly!
The problem is that we are all entitled to buy the spray collars, and yes I admit on first using one I got it very wrong and my gorgeous girl bloodhound legged it a half mile home. As time has gone by I have also seen the benefits can be huge, but remember dogs will inevitably revert to type given the right/wrong circumstances.
As the companion of two bloodhounds at the time in question, one of which had been trained not to chase deer using a spray collar, my rescue boy also required the same training. All appeared to have gone better than my wildest dreams. So much so that with both dogs off lead on one occasion we walked through a herd of fallow deer with the deer appearing unconcerned and continuing to graze less than twenty feet either side of us! I was gobsmacked, as were other passers by. Well all appeared rosy in the garden, as I felt relaxed and secure in the knowledge I wouldn't have a 'Fenton' moment. Life and experiences can really bite you in the backside when you become overly confident in your abilities as a trainer! Six months or so passed without incident and meeting the deer on an almost daily basis with no problem.
Until one day, when without warning a fallow deer stag came running from around a corner directly at us from a distance of less than fifty yards! Yes, all of my training and pompous preening about how well behaved my guys were around deer, flew out the window as if from a speeding car. Yes it was now game on, and there was not a thing I could do about it, their natural instincts took over and no amount of training other than maybe techniques we do not use in this day and age would prevent this! Well they had their fun for 10 minutes (which in situations like this is a very long time) and eventually returned like conquering heroes. No harm done to either themselves or the deer!
Why do we have obedience dog trials and why is there a winner? We have them because they are fun and amazing to watch a dog and handler generally working as a team. Sorry to be so obvious, but dog obedience competitions are won because some of the other dogs do NOT do as they are commanded by the handler, and not because the winner obeyed all commands but also on completion wrote an article on why he/she won the competition!
So in short, spray collars are magnificent for certain scenarios, although used incorrectly on a regular basis, the trainee dog can become neurotic and may never obey the command you are hoping he will.