I did the post-bacc program at Northwestern this past year (or at least most of it, as I had taken some of the pre-reqs in college). I thought it was fairly rigorous and challenging, since many of the students in the program had gone to Northwestern. I definitely had to push myself to do well, but if you put the time in, it can be done. I liked my professors for biology and physics, which is what I took there. Tests were challenging but fair, and labs were long but at least somewhat relevant to the class. The lectures were all but unnecessary, especially toward the end, but I still went to them. I think Northwestern has a good reputation and I'd recommend their program.
I actually took organic chemistry over the summer at Loyola, since I only needed one semester (plus I heard organic chem at NW is a total nightmare). The professor I had was very challenging, but the competition in the class was noticeably less hardcore. Apparently the other professor's class was a walk in the park. And the labs were kind of a joke - I thought NW's classes had better experiements and facilities.
All in all, I was pleased with both schools. I took all these classes right up to the MCAT last summer, and managed a 38 based largely on my preparation in my post-bacc courses (ran out of money to take an MCAT prep course). Plus Chicago is just the best city in the world, I mean seriously, it is. I should not have moved back to the dreary, rude, impersonal, hilly East Coast.
Feel free to pm me if you have any questions. I'd be happy to answer them!