I have been wearing my Vibram Five Fingers on and off for a month. I haven't been consistent, and haven't really worn them exclusively most days. I've had regular shoes on quite a few times, especially when at my standing desk.
Not a great plan for getting the feet stronger, and I could feel it. Thursday last week I was in the airport and on planes for a good part of the day wearing them. My feet were slightly tired, and on Friday I was tempted to wear my running shoes, but didn't. Instead I was out all day in them, walking through the National Air Museum, on hard floors designed to support the weight of large airplanes.
By Saturday, when I again didn't wear them because of the rain, I spent all day in college classrooms, on my feet, on hard floors. The run Saturday night was hard, and when I lay in bed, my heels were aching a bit.
I tried to step lightly, but caught myself pounding the floor a few times and realized that I should have worked up to this, with some longer days wearing the shoes. I need to back off a bit, and start wearing them around the house when I get home.