We love this. We were disappointed with the slack lines, so really happy this works for us. Our new house has a steeply sloped backyard, and ages 8 and 10 were really missing their swings. We have plenty of trees, but no low branches, so this is the perfect solution.
Although not recommended, I was able to install it alone. Just loop the top strap all the way around at the bottom of the tree and slide it up to where you want it. Tighten as much as you can, then continue assembly the way you're supposed to. I put it on a tree about 15" diameter.
Only one thing I'd change, if possible--lots of the slackline elements *could* be attached to this, but don't work well without TWO hooks. Make a sky stand with two hooks!