I have one and have used it for the pat 8 years or so. It is great for decompressing the spine and neck after heavy squats and great for general health. It was the bes $400 that I ever spent.
I can stay inverted for as long as I want know. I got the adapter and I use those special boots but before that I just put my feet under the bars and that did put some stress on the ankles. I generally stay inverted for 20-30 minutes a few times a week.