What questions should one ask to the former/current students of a professor before deciding whether to do PhD under him/her? Before deciding to do one's PhD under a certain professor, what questions should one ask of his/her former and current students?

Many students (including me) are not sure what exactly to ask. May be a community wiki here might be a general source for such students.

Please recommend phrasing of the question(s) also.

This is a related question, How to evaluate potential advisers on grounds other than their research/publications? but it is much broader. I'm looking for specific questions to be asked to the students. Also manner in which to ask :)

P.S. I have accepted an answer, but feel free to add more answers if you think they differ than the ones already given.
I think the most important is:


Are you happy with your PhD in general?


It covers a lot of issues, but usually you don't want to do a PhD in a place where students are unhappy and frustrated. The good thing is that if students feel really bad, they rarely hide it.

The next things are related to:


General contacts on the line student-advisor (How much contact and support can you expect, both for research and administrative stuff?).
Funding (Is it a problem or a non-issue, e.g. for attending a conference?).
Research (What the actual research looks like? What is the toughest part, biggest emphasis on, the most time-consuming part, etc?)
How much time does it typically take to finish PhD? Does it happen often that someone drops out?  


Other questions will depend of things you consider important. You may expect a lot of autonomy, or a lot of guidance. You may expect a very ambitious programme, or a PhD-life balance. You may like to teach, or you may like to keep it at minimum. You may be eager to travel a lot, or prefer to stay mostly in one place. You may be more into a particular branch of science or methodology, or into another...