LawDog has a post up on his blog about Tasers. Now, when he and I say Taser, we are speaking of a certain product, not using a generic term for "stun gun". A Taser fires probes a short distance- up to about 15 feet- which then deliver current into the target, incapacitating it.
I personally like Tasers quite a bit, but like every other tool, they have limitations. In the summer of 2005, I went up against a Taser. For, um, science.
From my perception, Tasers are likely to completely stop an assailant at least temporarily, but other options should be ready. In my case, I took the hit while charging, lost motor control, fell down, got back up, and attempted to resume the attack. This experience leads me to conclude that a Taser can be a helpful, but not complete tool for the self-protection toolbox. If you are forced to use one defensively, be prepared to immediately follow up with other options, whether that means running or transitioning to more permanently effective tools.
The after-effects of a sore jaw (from a friend who told me he- for some reason- "aimed at the upper chest") and small scar were not pleasant. The Taser darts are barbed, somewhat like a fish hook, and caution should be exercised with them. I would not suggest deliberately shooting anyone with them for training purposes, unless emergency personnel were standing by, on the off chance of some type of electrically-induced heart problem. In such circumstance, eye protection should be worn.
I'll talk about pepper spray sometime soon.