What makes the Tazer so much better than a handheld stunner?
A handheld stunner depends on pain compliance (and fear) to obtain the desired end results. The TASER takes the decision making process out of the bad-guy's hands. A TASER's electronic wavelengths are designed to run in sync with the wavelengths your brain uses to control your muscles. When you're taking that 5 second ride, your mind is entirely clear (aside from the pain), but you can not move. Most people lock up and fall over-- though a few of our deputies that I've witness being tased went straight up on their toes, locked straight out like lightpoles. The only thing keeping them up was the other deputies holding them erect. A good source for videos of the TASER in use is TASER International
Another poster mentioned the difficulty of getting both darts in the target--it's not actually that hard. There is a laser on the TASER that illuminates the point of impact for the top dart; the bottom dart is aimed 6 degrees lower that the top. Even if you only get one dart in and the opponent closes to contact range, you can apply the TASER directly to the opponents body to complete the circuit and still get the same effect.
Having sung it's praises, I still do not think it to be the best option for personal (portable) self defense. The M18 is as big as the M26 I carry at work, but I wear a duty belt. I do not think most people want to wander around with a giant TASER strapped to a duty belt during their day. Subject recovery once the current stops is almost instantaneous--it has to be since the tool was designed for compliance and law enforcement users will be giving clear, loud, concise directions to the subject and expecting him to comply by this point.
Spray will have detrimental effects on the target for around 20 minutes, is much cheaper to practice with, and has less of a media-stigma than the TASER. Considering how they've been vilifying cops using the TASER recently, I'm curious to see how major news outlets will respond to the first publicised tasing of a goblin by a non-LEO operator. There are always civil lawsuits to consider unfortunately.