I am thinking about this mic to go on a Canon 70d DSLR. Has anyone had any experience with it? The thing that is really interesting is it has headphone out.

Link

Polsen VM-180M DSLR/Video Microphone VM-180M B&H Photo Video

The thing that is really interesting is it has headphone out.
Does your camcorder not have a headphone out? Monitoring the SOURCE and not the recording device is of limited benefit, and can easily create a false sense of security. You could be thinking you are recording audio even if it isn't plugged into your camera.

Always monitor audio at the END of the chain. Monitoring it in the middle of the chain tells you nothing about the rest of the process.

It appears to be a clone of a Rode microphone. Most people would likely rather buy the well-known Rode product vs. an unknown clone. Especially when they seem to be essentially the same price.

The Canon 70D doesn't have a headphone jack, although there are hacks and work-arounds with various adapter cables.

I haven't used any of these mics designed for dSLR or mini video cameras, but the new Rode VideoMic Go can be bought with a furry windscreen for less money than the Polsen and is getting pretty good reviews. It requires plug-in power from the camera's mic jack rather than an internal battery as previous VideoMic models used.

As always, on-camera mic placement is sometimes a necessary evil, but should be avoided if possible as your main audio recording.