1. I'm not sure where the suggested 50% number came from, as I would put the number slightly higher, but I don't know a particular perspective. Students who take a course at one of the other colleges often will take more than one during their time there. Of course, the consortium is utilized by all students for more than just courses---lectures, events, conferences, concerts, study abroad, etc. are all ways my friends and I have used the 5 College system over the last four years.
2. Most students go to other schools for particular specialties. Languages, for example, often draw students to other campuses, as do arts courses. General education-type classes (note: Amherst does not have a distribution requirement, so there are not "general education requirements" like those at most schools, so I'm talking about intro classes) are usually taken on campus. It's important to note, too, that not all courses can be taken for credit at other schools (i.e. pre-professional courses, courses that are also taught at Amherst, etc.) Students go to the other schools to take advantage of courses not offered at Amherst OR professors with particular expertise. For example, as an Amherst senior, I'm currently enrolled in a foreign policy class at one of the other schools. I am completing the International Relations certificate, and foreign policy class was being offered at Amherst this semester. Similarly, my second thesis advisor is a Five College professor because she has a particular expertise in my subject matter.
3. Again, not sure on the percentage. A fair number of students do the certificate program--it's relatively simple. However, if your particular interest is neuroscience, note that the certificate has yet to be approved by Amherst. (Amherst does have its own neuro department, though.) See the latest list here: https://www.amherst.edu/academiclife/fivecollege
4. I currently take the bus to/from one of the other colleges twice a week. My only problem was on MLK day, when the bus ran on a restricted schedule, and I was almost late for class. Other than that, it has been fine. Because the bus takes longer than driving, I do miss lunch at Amherst and have to eat at the other college, but I have a free meal pass, so it is no added expense. It certainly is fastest to drive to the other schools, and often students will compare schedules are carpool. That being said, you can certainly plan your schedule to get back and forth from the other campuses without issue.