This is a good thread to resurrect now, given that the question was just asked again and people are starting to interviews this month.
What has already been said is very good advise. I would echo that the more specific questions you can ask, the better. Asking, "Are people happy here?" may tell you a little, but asking specific questions that get at the things that would make YOU happy in a program is going to be much more helpful.
For starters, you might want to ask a general question like "How many people have left the program in the past couple years before finishing, and why?" because that's a good thing to know. But then I would get more specific. For me, the things I wish I would have asked about include (and each person's list is going to be different):
1. How many hours a week do you ACTUALLY work on specific rotations? What time do you generally leave the hospital at the end of the day (because there's a big difference between 3 and 9...)?
2. How many of your rotations violate the 80 hr work week (or the 30 hr shift length)?
3. Do you make it to noon conference regularly?
4. Does the program make a decent effort to take people's scheduling requests into account when making call schedules (or are people left on their own to find coverage when they're the maid of honor in their best friend's wedding....things like that)?
5. How much time do you spend with patients compared to filling out paperwork, calling consults, making appointments, looking for charts, tracking down results, calling the lab....and so on (this one is big...)
6. (If your program has a regular continuity clinic experience) What is the preceptor:resident ratio in your clinic? Do you feel like there's enough preceptors so residents (and their patients) aren't waiting forever to present their patients?
7. (This one probably applies mostly to IM and peds) How many rotations are spent working mostly with subspeciality patients whose care is dictated by the subspeciality team and then carried out by you (making you little more than a glorified secretary)?
8. (Also mostly applies to IM/Peds/FP) How many of your ambulatory months also have night/weekend cross-coverage responsibilities, and does this extra coverage take away from the ambulatory experience?
9. A related question....How much of your elective time is truly call-free and how much has night/weekend cross-coverage responsibilities?
10. Where do people live, and how do they get to the hospitals? If people want to buy a house/condo are they able to?
11. Give me an example of how the program has responded to resident feedback in the past few months.
12. For those with spouses/partners/children: Does your family feel like you spend enough time with them?
13. The last time you were sick, did you feel like you could get coverage, or were you hesitant to use sick call until you were nearly dead (and if so, why)?
And finally....14. How much does the program provide for the little things? (providing enough $$ for meals on call, providing parking for free or at a reasonable cost, providing scrubs, etc)
Most of these questions are best asked of residents, but you have to find residents who are going to be really honest with you (and preferable more than one, because people are going to give you varying answers). Most program directors are going to present the best pictures possible of their program. Some rare programs will lay out the limitations of their program, too. With a few exceptions, they're not the ones you should be asking the majority of your questions to. You already have access to (and have probably already read) all the information they will give you on interview day. Residents are the ones who have a lot of the info that you really need to make a decision. Trying to do a rotation at your favorite places isn't a bad way to get this info, either.
Your goal on the interview trail shouldn't be to find the perfect program, because there aren't any. Every program has it's limitations. Your goal should be to figure out what those limitations are for each program, and then decide which ones you can live with. Residency is a huge commitment of your time, whether it's for a 3 year FP residency or 7+ years for surgery, and you need to know what you're getting into. Know what's important to you before you get into the interviews.
In the end, a lot of programs will probably start sounding the same, and I'll tell you--apart from location, what's going to set them apart are the little things, and believe me, these things will matter more to you a year than you may realize right now. At least that's how it was for me. Your "gut" feeling about a program should guide you, too. You'll likely be able to pick up on how happy people are at a program without asking them.
Good luck to everyone!