I have one I bought many years ago. At that time there were two types on the market. One had a bigger needle and larger tacks and was often accused of leaving bigger holes. Mine has the finer needle and smaller tacks.
It worked really well for me, but the problem I always had was finding a suitable place big enough to baste on. At the time they sold an egg-crate type grid you could slide under the area to be basted, so the quilt sat on that and gave you room to insert the tack without hitting the table underneath. I didn't like that, as the quilt sandwich always shifted on me. I did use the gun with my homemade quilting frame and it worked well that way. (This is a frame of 2x4's that can be clamped together. The quilt is stretched out completely. Takes a lot of space, and sags in the middle, but worked fine for my twin size quilts and smaller.) I think it would be impossible to use a gun on a quilt laid out on carpeting; the tacks would stick in the carpet. I never tried it on a hard floor; might work well that way. I'd test it first, though, in case the plastic tacks are shot out hard enough to mark the floor.
I don't know why I wrote all of the above!!! I don't see any reason to use a tacking gun now that basting spray is widely available. Basting spray is much faster and there is no problem with thread getting caught on a plastic tack when quilting.