How effective is the Onzow Zerodust?

It is pretty good for its price. Well, I think, the main advantage compared to fluids is, it is safe. With fluid you should be careful, some (not all) can go into the cantilever with all negative aspects...and not to much pressure to the cantilever while cleaning...
I use the Onzow in combination with an Ultrasonic cleaner from audio Technica. That's the best combination for me. No force to any part of the cantilever and always a very clean needle for top reproduction.

I am a convert to the Magic Eraser (the cleaning product) stylus cleaning method; there is an Audiogon thread by Dougdeacon describing the method and how it works. It works wonders on my carts. One needs to make sure not to be too zealous with it. Patience is key.

I have bought old gunked-up stylus carts which were cheap because they were so gunked-up, and made them look as if they were new, and thoroughly enjoyed them.

I use the Zerodust to clean the little 'bunnies' which sometimes end up at the end of the stylus. For any serious cleaning, I use the ME.

A ZeroDust will do no harm, but by itself it won't clean a stylus completely.

Liquid products work better, but they pose some risks and are not quite as effective as the Magic Eraser.

The ME also poses some risks (so does any stylus cleaning method) but many hundreds of users believe it's the most effective stylus maintenance technique, and it's virtually free. I've been using it regularly for six years, on cartridges costing up to $5K. All of them look as clean as the day I took them out of the box, even under 200X magnification.

This thread contains a sampling of happy users' recommendations, plus a link to my original post explaining how to use. There's also a link to a Wikipedia article on the material itself. A search here or on VA will turn up many more.

I use it on every play, then once a month or so, I do the disk doctor liquid.