Pepper spray is one of the best self-defense tools when used correctly. Because it is such a low level of force and unlikely to cause serious injury it can be used earlier and with less provocation than other defensive tools and techniques. At the point at which you think an encounter may become physical you can apply pepper sprays.

Pepper Spray it not a guaranteed fight stopper.
Firearms aren't guaranteed fight stoppers either, so this isn't much of a negative. Have a plan for transitioning to a better weapon system, and most of all keep moving.

Pepper Spray doesn't quick draw very well.
It needs to be handy in order to be useful. If you toss a can into a purse or backpack it is not going to be in hand when you need it. I carry it in my dominant side jacket pocket, and get a hand on it as soon as I think a threat is possible (not likely, just possible.)

Pepper Spray is better as a surprise.
Warning somebody that you will pepper spray them gives them a chance to mentally prepare. Motivated people have no problem fighting through pepper spray if they have a goal. Many training courses and law enforcement academies have a number tasks that subjects must complete after being sprayed and before they are allowed to decontaminate.

Flip-Top Safety is the way to go.
Most of the major manufacturers use a flip top, which is far superior to the switch-type safeties. You need something the is easy to access and orient towards the threat.

It's all "hot" enough.
Getting a eyeful of the weakest stuff on the market is plenty painful. All pepper spray hurts. I care more about the container, the safety and the spray pattern, than I do about the relative 'hotness', dyes, and other stuff the manufacturers use to differentiate their product.

Know your spray type.

Streamer/Foam Types:
Shoots a narrow stream.
Generally has the most useful range.
Produces reduced secondary respiratory effects.
Requires the most accuracy
Yields multiple/longer uses from a single canister.

Fogger/Cone Types:
Shoots a wide cone or cloud.
Shorter range.
Produces maximum secondary respiratory effects.
Yields fewer/shorter uses from a single canister.

Pepper Spray is a distance weapon.
If you are going to spray somebody it is better to do it before they have a hold of you. Most sprays will push the product out to at least ten feet or more. Use the distance to get away.

Bystanders may feel secondary effects.
This will include uncomfortable burning of the eyes and maybe difficulty breathing. If you are using pepper spray in a crowded location you will probably not be too popular with the bystanders.

Get Training and Practice.
It takes a little bit of skill to get your product on target, especially while moving. Most manufacturers make inert versions of their products to practice with, although stupidly the inert versions cost as much as the real thing.