I have one, and it does work. They tell you you need to zap the pimple as soon as it starts to emerge, and they're right. Twice in 12 hours and it subsides.
It doesn't work on cystic blemishes, although I tried it on one and it didn't get worse as I would have expected it to. And it's not a substitute for a good skin care regimen. It's ideal if you just get an occasional small breakout.
I bought one about 2-3 years ago, and I didn't notice much of a difference. I'd have to catch the zit really early, when I could just barely feel it under the skin, and it still wasn't enough to prevent it from coming out. I do think they were a less red than usual, though, but that's about it.
But I've never been able to get rid of a zit once it's formed. None of the products that so many people swear by worked to dry it out. I would just have to wait for it to disappear on it's own.