I recently bought one of these based on Youtube reviews:
I was interested in it for two reasons, one, I have used the Bodyblade for conditioning, isolating power chain muscles, and rehab purposes and I really value it, and this thing is basically a small, fist-sized Bodyblade:
Also this device is similar to a makiwara, in that the purpose of it is to help you develop your power chain and time your punches for maximum impact. So, having had positive experiences with similar technology, I figured i'd give it a try.
I have to say that so far I am very impressed, It took a bit of time to really understand how they recommend using it, but after watching a few videos I got the basic idea. So far I have used it to try to isolate the body movement of hook punches, i'm seeing results in just a couple days.
So, I don't think this thing can magically make you a dynamite puncher, but I think it can be used in a considered way to hone in on improvements to technique. It's also around $20.00, so not a huge investment.