I initially planned to pursue an MS in computer science after undergrad, and had gone so far as being accepted into a program, and attending classes for nearly the entire first semester.
I bailed out at the first semester when I considered the opportunity cost involved. I'd be spending an additional two years (or a little more, depending on how much I could afford all at once) in a program that was 95% theory, while missing out on two years of learning practical work through employment.
But, I think a 5 year BS/MS program is different. Finishing in five years even with just a BS isn't out of the ordinary anymore these days. You haven't graduated yet, so you still are 100% within the "in school" mindset. And it's only a year out of your life instead of two.
I don't regret dropping out of the two year MS program, but I do regret not having pursued the 5 year BS/MS package when I would have had the chance. I would have gotten the additional education, but it would have cut that opportunity cost in half.