I used an Anderson Tree Sling (one of those "original" designs from decades ago) and I actually like it a lot. It is very easy to pack, and very versatile. Like anything, it takes practice, but I can get into trees with it, that are not possible with other settups. I have taken many deer out of them with longbows.
When I was younger, this was my first treestand option purchase and served me well. There are drawbacks....the kneepads and shooting to the right side, but almost anything has drawbacks. The worst one, I think was the pressure of standing on steps for hours. Other than that, I feel safer in one of these than in my other stands, as I'm firmly attached to the tree. Of course I've hunted some windy days that made me wish I were'nt.