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My acceptances came within a few days to a week of the interview, while my rejections from Dartmouth took 2 weeks and Princeton took 6 weeks.

hey! I totally understand that, kudos to you for sticking to your guns with your programs and reconciling with all of that, it sounds like you know what you're doing! Well at least, totally more than me, I've been running around like a chicken with its head cut off since like November. Wishing you the best of luck! 
I've heard fairly good post-interview acceptance rates for PIBS, generally around 2/3+ are accepted from what I've been told. A friend of mine is a 5th year student in the Biological Chemistry dept, and when I mentioned my interview he said that an invite "almost guarantees" acceptance. However, I think he was exaggerating a bit because he was excited for me . I imagine it might also depend on the program you intend to join if you matriculate, and how many spots they need to fill, etc. I know for example that a higher number of applicants than usual applied for MCB, and therefore there were more rejections on that front. I applied with neuro being my priority interest, and I was called by the assistant program director on 12/15 before invites were sent out on 12/18. 
His other advice/insight was mostly this (some of which was also echoed by commenters here) - the admissions committee takes the opinions of the faculty you interview with, and the student hosts you have, into consideration. I believe they write reports on you which are then forwarded to admissions. I've been told the interview weekends can be pretty overwhelming (~100 people interviewing for PIBS, and UMich is a big school with a large campus regardless). I was told to make sure I'm engaged and interested the whole time, and to not mention other schools as being your first choice, etc. They want to sell you their program just as much as they want to make sure you're a good fit, as they're already paying money to get you up to Ann Arbor and hosting you all weekend. From what I can tell, they'll email you your ~personal itinerary~ 1-2 weeks before the interview, with the names of the 5 faculty you're interviewing with. My plan is to make sure I dedicate some time to researching each one and making a list of questions/things I would like to bring up with them in interviews. I have a little notebook I might dedicate to this purpose, worst case scenario I look like I've done my research, ya know?
The other thing I would say is dress warmly and comfortably! Not sure where you're from, but I'm a Michigan native, and it can be pretty damn awful in the winters. Wear good shoes and bring a warm coat, and check the weather before you pack. Act like yourself and have a good time, but also be on your best behavior during department lunch/dinner outings, don't get drunk or do anything like that haha. I have a few friends who were accepted through PIBS last cycle who I plan to reach out to soon, and my PI got his PhD through their neuro program - he's a pretty young guy and might have some interview advice specific to UMich that could be useful. Once I'm back on campus he'll be helping me do some interview prep and I'll make sure to thoroughly interrogate him and report back haha. Overall I think the best thing you can do is enjoy yourself and have a good time while making sure you're engaged and clearly conveying your interest to faculty during your one-on-one appointments. 
Best of luck! If you happen to be interviewing on 1/25-1/27, I'll be there too! 

Just got an interview call from University of Michigan PIBS!! So excited 

Just got an interview call from University of Michigan PIBS!! So excited 

Thank you! I applied to the Molecular and Cellular Pathology program. For what its worth, I called PIBS yesterday asking for a timeframe during which they will be sending out interview invites, and the lady I spoke with said they'll send them out before the holidays. I know thats not exactly helpful but I'd maybe keep a lookout until the holidays. Good luck! I hope you hear from them. 

Hey everyone, best of luck with interviews and the whole admissions process. I know this is a very stressful time, but remember that you have worked hard to be where you are now and that hard work will pay off. 
If you don't get an interview at your dream school, don't worry about it. I know plenty of people who ended up at a school other than their top choice and a year or so in they are loving it. Everything will work out, and at the end of the day, it's all about how hard you work. If you're going to be a good scientist, then you're going to be a good scientist, regardless of where you do your PhD. I know people who get Nature papers at lower tier schools and people who never publish at Harvard. It's up to you to make the most out of your experience wherever you end up, so go nail your interviews regardless of the program and be the great scientist you know you can be. 
Good luck everyone! 

Wisconsin-Madison Genetics adcom called to invite for an interview! Also got accepted to the direct entry Ph.D. Molecular Genetics program at University of Toronto!! I'm officially going to graduate school no matter what!

Just got an interview call from University of Michigan PIBS!! So excited 

It looks like the only interview invites have been by phone so far. Years past indicate that they also send e-mails and it's mostly spread out over a few weeks, with some calls as late as January.

Though I'm with you on feeling a bit pressured and anxious that people are hearing back, it is early and everyday more and more schools are sending out invites. Though it's next to impossible, relax and trust in yourself and your application. Good things will come soon enough. (It's helping me that I haven't really seen any of my schools on the results page yet.)

literally in the same boat - fingers crossed for the both of us!

Same!! Just over here refreshing my email and the results page way too often. Good luck!!

Just got an interview invite from Ohio State Biomedical Sciences Graduate Program! Very excited! I look forward to the interview and meeting everyone there!

Having gone through the process twice I am very curious to see how your application will fare. It is difficult to predict not knowing the rigor of your undergrad curriculum. That being said some of the programs seem very specifically focused e.g. immunology and I know some of them like UC Denver I applied to with stronger stats than you and did not get an interview. I also did not have any publications though so who knows; there is a lot of chance and luck to this whole process. I will be interested to see how you end up doing! Wish I could be more help!