Is this tool any good?

While it may level and polish frets in relation to their nearest neighbor, it won't be able to deal with any unevenness in the playing surface over the entire length of the fingerboard. It's probably fine for a touch up dressing once in a while, but I'd recommend a pro for optimum playability.
Also, I fail to understand how a "longitudinal" motion is going to crown, or restore the peak, to any frets. It sounds a little hokey to me.
If you get one let us know how it works, it's certainly affordable.

 

I thought the same thing when I looked at it.