A few years ago I bought one of those for my apt. I did a max of 3 per set. After 2 months, I'd done roughly 2,000 pull-ups and had to stop for baseball season since it gave me a little tendonitis in one of my forearms. I had a log on it somewhere I can't find now, but it was a great experience. Do some every day and you'll be shocked at how your body can respond to the increased demand.