We have one that we've had from a previous dog (that it did nothing for). When Coby was little, we tried it for barking. Didn't work. So we did pennies in the can, which worked for a while. Then he got "immune" so we switched to the spray water bottle. That worked for a long time, but then I swear Coby just started barking to get a drink from it! So we tried the ultrasonic remote again. Works like an absolute charm!!
So in my opinion, for Coby, training is like shampoo... you have to switch brands every once in a while or it loses it's effectiveness.
I've read bad things about the bark-activated ones. We just keep it handy and press the button.