I attended in the spring a few years ago. For some students, the program was a great time but not terribly necessary (mostly the folks from elite schools) but for those from small schools with very limited options (like say places with math depts with only 3-4 mathematicians and very few math majors), the program is a blessing in that there’s the opportunity to study a wide variety of things and with plenty of other students doing the same. It isn’t the case with everybody, but the level of the coursework was also much higher at BSM than it was at my home institution and that offered me the chance to get used to things that were closer to the graduate level than what was offered at my home institution (there were no grad courses where I attended). I’m finishing up my masters and will be transferring off elsewhere to finish my PhD but I entered graduate school comfortably and found my background to be more than enough for the coursework and I really believe that my experience at BSM helped with that whereas many of classmates who came from similar institutions without BSM had a rough semester or two from the adjustment to graduate school.
Most if not all of the BSM instructors would serve as excellent letter writers for graduate applications. Some are even willing to tell you about their experiences from graduate school. Just get a group of students to go to dinner together somewhere and email a prof to go with you. Some of my friends have heard quite a few stories from remarkable mathematicians as a result.