I have one of the GloBee tachs also. I haven't used it much, but so far I'm satisfied with it's performance. I have noticed, though, that it gets better readings near the prop hub than at the tips, as others have testified to as well.

I figured that this was because the blade has a wider chord near the hub which makes a much larger "shadow" than the narrower chord near the tip.

Also, I've only used it on APC props so far, I don't think I've used it on any Master Airscrews yet. Maybe the black color of the MAS props will make it work better out at the edge of the prop disc.

The weather forecast here is looking like it might be quite conducive to flying activities this weekend, so I'll have to try it on on some different props.

Like I said, though, I'm satisified so far with the unit's performance. Good stable readings, just have to hold it nearer the hub than the tips on the prop disc. So far, that hasn't been a major problem, just an idiosyncrasy to work around.