I've used a similar mist maker before. They're okay, as long as you have the right expectations. The effect is fairly small, so your audience would have to be pretty close to appreciate the effect. Don't expect mist to billow out of a large cauldron and flow across the floor or anything. Placing it in a small bowl on a table would be the most appropriate usage.
I tried puting mine inside a pumpkin (the device was sitting inside a small bowl of water). And while a bit of mist drifted out of the face of the jack o' lantern, it wasn't quite enough to make it worth the trouble, in my opinion.
In order to work properly, the device needs to be submerged in an inch or two of water. Which becomes one of its drawbacks because as the water dissipates, you'd need to refill the container to the right level to keep the effect going. Especially if you have it in a small container that doesn't hold a lot of water to begin with. And personally, I didn't think it was worth the trouble to monitor water levels when I'm already running around like crazy on Halloween. Also, because the device creates mist by vibrating a small ceramic disc, if it's too close to the surface of the water, it can spatter the liquid and make the area all wet.
So as I say, for a small effect that you're viewing up close for a limited amount of time, it can be pretty neat. But I like to go bigger with my misty effects, so I'm more likely to use dry ice or a fog machine.
Here's a video of a mist maker in action, which should give you a good idea of it's effect:
Mist Maker/ Fogger with LED Lights
