I have used one and once you get the hang of it, IMO they're great.

I had difficulty getting the hammock stretched tightly enough, until I took a survival course from Mors Kochanski, in the course of which I was introduced to the turnbuckle (or parbuckle?) knot. Basically, you tie a figure-of-eight knot in the line, but don't feed the end of the line all the way through; so you have a loop sticking through one end of the figure-of-eight. The end of the line goes through the straps around the tree, and is then fed back through this loop. The loop acts like a pulley and changes the direction of the pull; the result is, when you pull on the line to tighten it, you are pulling it toward the tree rather than away from the tree. This makes it much easier to tie off the line without it slipping and causing the hammock to sag.

I also found that if you get a small foam pad and stick it inside your sleeping bag, it will stay put far better than if you use a full-size pad and put it under the sleeping bag. Don't try to use a HH without a foam pad or other insulation, your bum will get cold.