How important are college rankings when choosing a college?
This is a particularly loaded question, but here's the skinny: It depends. First it depends on what kinds of programs you're looking at and what kind of career path you have. If you're considering advanced degrees and continuing education in the same field, rankings will often matter quite a bit. E.g., if you want to be professor of philosophy, then you need to get into a top philosophy Phd program. In order to get into Phd program, it will help if you went to undergrad at an institution known for its top philosophy program and faculty. This is not so much because you'll end up a better philosophy student so much as the networking opportunities you'll have with other movers and shakers in the field. If you're looking to work in field that's not "specialized" (read: not a hard science) then your degree and the relative ranking of the school will likely matter much less. If you're considering a graduate school right now and are looking at an MBA, JD, Phd, MD, DO, or the like, rankings matter a lot!
While I haven't give you specifics, that's in large part because it depends on where you are currently and what type of work you want to do. Don't get me wrong, if you graduate from an Ivy League caliber school, you'll probably be in a better position that someone who graduates from and unranked and unknown school. But that doesn't mean the latter won't be as or more successful than the Ivy League grad, it just means he or she may have to work a bit harder.