I wrote an article on using pepper spray against dogs a while back:

http://www.crazyguyonabike.com/doc/dealingwithdogs

I use the Fox Labs spray now, since in my experience the Halt! is not always strong enough to deter all dogs. Also, I have heard anecdotal stories of someone with an older can of Halt! spray, and their dog was licking the nozzle - it apparently tasted good! Again, that's third hand anecdote. Point is, pepper spray might very well lose potency over time, and also the can might slowly lose pressure, so don't keep a can for years. I try to change mine every year or two.

I use the Fox Labs spray now - it's more expensive, but I also don't use it as much. There are links to a good vendor at the end of the Dealing with Dogs article.

Someone mentioned stream vs fogger. I have used both, and they have their advantages and disadvantages. The obvious advantage of the fogger is that you don't need to aim it as accurately. The fine mist will spread out over a larger area (in the rough direction you sprayed it, hopefully). That last is crucial - "hopefully". On a bike, wind is often a factor. While in theory you might use less spray with a fogger, since you're not having to aim/miss/aim/miss trying to hit the eyes/nose/mouth, on the other hand the fog disperses on the wind much more and if the dog is on the upwind side of your bike, you might just end up spraying yourself, your panniers, or your partner etc. So unless it is dead calm (and I'm going very slowly), I prefer the stream type. It's true that the stream is harder to aim, at least initially. You just have to practise. Over time you do get better, guaranteed. Also, you can buy inert units for training purposes - again, links at the end of the article. It pays to practice. You'll always waste it a bit getting the initial shot and then zeroing in on the eyes/nose/mouth, but it is definitely a skill worth learning.

I like to carry a spray that has wider application than just dogs. Unlike some others, I do feel that pepper spray would be a good defensive weapon to have against "bad people". The arguments against this seem to go much the same as those against using it for dogs - "I've never had a problem, therefore it's not worth carrying". Or, "You are just going to piss them off" as Pete alluded. I respectfully disagree - I've actually been attacked by dogs, it's not a pleasant experience and it does really help to have an effective non-lethal deterrent. Also, most dog owners understand the use of pepper spray for defense, they know it's not permanent and they probably won't come after you with a shotgun if their dog chases you on the open road and gets sprayed. As for using it on people, I honestly like to have a weapon that doesn't actually have much risk of injuring the other party permanently (there is always the risk of an extreme asthma type reaction, I guess, but then again we're talking about extreme situations where you are being harrassed or attacked, so I'm assuming you have to defend yourself). Here's the thing: You simply cannot anticipate every situation that might occur. You might go your entire life without ever encountering any kind of extreme situation. Or, you might encounter it tomorrow. Carrying a couple of cans of pepper spray on your handlebars (remember: It has to be instantly accessible, no use at all if it's stowed away somewhere) - this is not very onerous at all, the cans weigh next to nothing and they give a good feeling of "last line of defense". Better to have than not have. Finally, you could, in a pinch, use pepper spray against a bear or other large animal (cugar?). It's true that the proper "bear sprays" have a much longer reach, 25 feet or more, but again, we're talking last line of defense here. You just never know, is all. I also carry a fairly serious folding knife that is readily available.

The key thing is to be ready, but not overly paranoid. Just because you carry the "weapons of last resort" doesn't mean you have to have the mindset of the paramilitary nutjob. It just means you're ready for more situations than you would be without, and if the cost of carrying (in terms of weight, hassle etc) are not significant, then I say "why not".

Pepper spray is definitely most useful against dogs. I don't spray every single dog that comes my way - it's certainly true that 90% or more of chasing dogs are either friendly or else just out for a chase, not intent on making a kill. You can often tell by the body language, number of dogs, type of bark and so on. When I see a dog coming out to meet me on the road, I get the spray ready, and greet the dog, keeping eye contact (don't behave like a victim, dogs respect alphas). On the other hand, some dogs respond aggressively to challenges, so it really has to be on a dog-by-dog basis. In any case, to be honest, even if a dog is being very friendly and just wants to play, it really shouldn't be running out into the road and running around your wheels. It could (and has) very easily cause serious accidents. You don't want to be weaving around dodging the dog, you might end up under a car yourself. And often you'll be going uphill, and fully loaded you probably won't be able to make a run for it. So having the pepper spray to deter Fluffy from taking a bite is a Good Thing (TM) in my book.

Neil