I used one until I found basting spray! Basting spray is much faster for me and easier on my back. Before using the gun, I used safety pins. For me, the gun was easier than pinning.
If you buy one, be sure to get the right kind; not all guns sold for quilt basting are good ones. The best type has a small, narrow needle and uses smaller tags that hold the quilt sandwich together snugly. Many of the complaints about guns are from the wrong kind of gun.
My gun is from Avery Dennison and uses 1/4" fasteners. This is a good gun.