Well, you are really asking two different things here:
1.) The title of the thread is "Do you let others USE your 'Book?"
2.) The poll on the other hand asks, "Do you let others BORROW your 'Book?"
To answer the first question, yes I have let other people use my iBook. I have been right there next to them, though.
To answer the second question asked by the poll, the answer is a resounding NO. I would never let anyone borrow an expensive item like a laptop computer.
I am very protective of my high ticket items (like my computers), and while I don't mind if someone wants to take a look at it, or use it for something quick while I am there with them, I would never let anyone borrow it for any extended period of time.