I'm in the BA&Sc, and I think it's a good fit in a couple situations:

1. If you're equally interested in two areas, one in arts and one in science, AND you are firmly committed to considering things in an interdisciplinary sort of way. If you're even the smallest bit less committed to one side than the other, you should just do a BA or a BSc and minor in the other area. The major programs in arts and science provide more than enough space for this.
2. You are interested in doing one of the interfaculty programs, e.g. cognitive science, which you can only major in as a BA&Sc student.

If you don't fall into one of those two categories, you probably shouldn't do a BA&Sc.

As for what being in the BA&Sc is actually like...

• I really like doing an even blend of arts and science courses. They have very different rhythms to them - arts courses mostly entail reading a lot, thinking about it, coming to class ready to discuss, et cetera. Science, depending on what science you are looking at, pretty much falls into either math or memorization.
• Course work tends to be structured very differently in arts compared to the sciences as well. In a science course, you are likely to have somewhere between 4 to 10 assignments over the course, combined with one or two midterms and a final. In arts, you're likely to have a couple writing assignments, maybe a couple in-class "quizzes" or tests, a final paper, possibly a final. Evaluation is much more fluid in the arts. Science is a lot of smaller bits throughout, while arts is weighted towards the end, typically.
• Things get kind of heavy on the multi-track option though, and I'm thinking biology and polisci might be a really heavy combination. I'm not very familiar with either field, but from what I have heard (and if anyone else knows more, please chime in) biology is very memorization heavy, and polisci has a ton of reading.

I transferred into ArtSci in second year and had to make up my freshman arts requirements. That, combined with having to do BASC201 (luckily that was a really amazing, fascinating course) means that I get a grand total of 2 elective courses in U1-U3. But I love both my majors, and I get a fair amount of flexibility within my arts major, so I don't mind. I am kind of jealous of some of my arts friends who get an insane amount of electives, though. There are a lot of things I'd love to sample that I simply won't have time for.

Finally, you should make sure that whatever you do in a BA&Sc degree would meet the requirements for your med school application. Again, I have no clue what you need for med school. Probably at least first year sciences. Not sure where you are applying from. If it's from Quebec, CEGEP should have prepared you for that, or maybe you have AP credits or you did IB. Otherwise, you will be doing them in U0. If you do the BA&Sc, you have to do three arts courses in your freshman year, so you don't have time for all the foundational sciences and maths. If you can't squeeze all the stuff you need for med school into your first year, you will have to make it up in electives later.