I wouldn't normally volunteer, but given that the video and experience can be used in court, it's really not a bad idea. There are no after effects, it lasts for only 5-seconds (less if you really can't take it), you can articulate it's effects, and you get a good story.
Briefly, it's bad in that you cannot move anything between the electrodes. There is pain involved from the current hitting your skin (arcing) and from all the nerves being fired by the current, but all the muscles between the electrodes are clenched so you cannot move them. Essentially, because a bunch of muscles won't move, you're helpless and have no balance until the current turns off. Once the current is off, there are NO after effects (if you don't count electrode related "contact wounds"). But you can't do squat while the current is on aside from feel pain and think (according to some), "why did I volunteer for this?" The X26 is more efficient, more "painful" and more localized. The M26 feels more like they're using much more energy (they are) and the effect is a more spread out pain/clenching that feels more like "the Jolly Green Giant shaking you." The energy and pulse rates between the two models result in the same incapacitation, but via slightly different (efficient) effects.
Volunteered. F-r-i's advanced classes include training in the appropriate use of a Taser. This is something that is also part of Taser International's normal training (i.e. vounteering to experience it). Additionally, as this is part of a personal preparedness class, it also means that should someone threaten you with a Taser, you can articulate in court why you took certain actions.