They're allegedly fairly effective (the claims are what, 90 or 95%?)... if both barbs hits. If it's not effective, you have no recourse. There are no effective failure drills.
Reloading simply takes too long, and as previously stated you only get one shot, maybe two depending on model, between reloads. If the very first shot or two doesn't work, you're in trouble. By the time you reload, an attacker can close the distance or get out of range.
They do have a good safety record, but I'm not convinced the mode of action is completely safe. Most other weapons, like batons, can be used in a more or less vigorous manner depending on the offender. A taser is a one size fits all solution, and if there's any chance it can kill, it can do that just as easily for uncooperative suspects for minor offenses (where the risk is not really justified) as it can when subduing violent suspects (when the risk of taser-induced death or serious complications might be worth it).
The cartridges are expensive. How does anyone expect to practice with a taser?
Electronics and batteries can die.
Aim is fairly critical because both barbs have to hit, and the separation (of the X26c - civilian x26) is 8 degrees (according to the website). At 15', that's 2.1' of separation (2.0927... and that's linear distance, NOT arc length). I don't know if the dispersion is horizontal or diagonal or vertical, but hitting a moving target with both barbs is something you might want to practice... and practice would be very expensive.
Finally, is it a deterrent? It may hurt, but the attacker knows he will be okay afterward. There are also several things I can think of an attacker could do to minimize chance of getting tased while closing the distance and getting within arm's reach.
I have no problem with police using tasers against a violent or suicidal individual. In particular, when there's backup available, it's the ideal weapon to use if police can get into taser range. I have no intention of getting a taser because I think for a single person, for self defense, it's not as good of a weapon or deterrent as the company would like people to believe.
“The egg hatched...” “...the egg hatched... and a hundred baby spiders came out...” (blade runner)
“Who are you?” “A friend. I'm here to prevent you from making a mistake.” “You have no idea what I'm doing here, friend.” “In specific terms, no, but I swore an oath to protect the world...” (continuum)
“It's a goal you won't understand until later. Your job is to make sure he doesn't achieve the goal.” (bsg)