Has anyone used one of these?

 

Owners here have used it, and some of them were happy while others were not. I think it depends on which bike it's installed on, but there are lots of alternatives out there. It depends on your set up and mods and what you're trying to accomplish.
Doing a search on tuners will bring up a ton of info. Tell us what you have and you'll get some ideas from the members.

 

For a canned tune, they work good. I've had one on my bike for over a year now without any problem.