First off, don't confuse the Taser and the new Ait Taser. They work on different principles.
The old Taser was a pain compliance weapon. It used darts or was a contact weapon. It means that you either have to be very close, or you have to hit with both darts. It was not always effective (remember, they used it on Rodney King and it didn't even slow him down a bit).
The new one is effective, but both darts have to make contact with their target. If they don't both hit (and have more than a few inches of separation) they won't work.
If they do hit, the current overrides the central nervous system, which is much more effective. You automatically go into a fetal position on the ground, screaming like a little girl. The pain is just a side effect, but it ain't fun.
As far as a personal defense option, I do not think the technology is mature enough, yet. If a dart misses, you can pop out the cartridge and use it as a contact weapon, but it just isn't foolproof enough yet for personal defense. I think that it is currently only suited for limited police tactical applications, when you have other people present covering with other use-of-force options.
For personal defense, I would stick with a good pepper spray backed up with a quality handgun.