I didn't think I would like this, because I thought it would be one of those tools that is more trouble than the task it is trying to simplify. But I actually now consider it to be a "must have."
It consists of a very sharp series of blades which are powered by rolling the device with your palm along the countertop. The blades are housed in a clear plastic case with a trap door through which you insert the garlic cloves. It is very well-designed in how it fits in your palm, and the wheels are very smooth. It is also very easy to clean, simply rinse and allow to air dry. The one thing that is a bit of a hassle is removing the minced garlic from the housing once it's been chopped - I use a butter knife or paring knife to just run along the edges and between the blades to lift out the minced. I consider this modestly tedious task equivalent to the tedium of the sticky scraping off a cutting board that regular garlic mincing requires.
The best part is that this creates garlic that's minced much more finely and evenly than I could do with a knife. Another boon? This is sold by Chefn, a Seattle company.