I started them about a month ago while rehabing an injured knee. Awesome movement. Previous to injury, I would do barbell squats regularly with respectable weight. Doing single legs squats certainly humbled me. I could barely get 8-10 first time. They require the use of a lot of stabalizers, and are great for lower body mobility. I think they're a fantastic exercise to add to any lower body workout. I've decided to drop barbell squats for a bit, as they were causing some knee pain. Hip mobility problems are a bitch. Dropping barbell squats was tough to swallow, but I've not noticed any atrophy, and I'm sore in places I was never with barbell squats.
Long story short: Give single leg squats a try; good stuff.