How important are college rankings when choosing a college?
College rankings are interesting to look at, but more important than the overall rankings would be the reputation(s) of the department(s) in which you might choose to major. Some schools which are not at the top of the overall rankings have outstanding programs in certain areas. Quite negative rankings could raise "red flags" about specific institutions. In those cases, you would want to research those schools thoroughly to alleviate any concerns you might have.
If you do feel compelled to consider rankings, the criteria on which the rankings are based might be more relevant to you - things like "Freshman Retention Rate", "Graduation Rate", and so on. You can also get an idea of the SAT/ACT range of most of the schools on the ranking list to determine whether a college/university would be a realistic choice for you based on your own scores. There may be other ranking criteria that are of interest to you personally.
A number of institutions, some of them with excellent reputations, have stood up against the ranking systems, feeling that they inordinately sway student and parent opinion without providing a full picture of the schools involved. Keep this in mind as you use the ranking systems as part of your college research plan, and be sure to use a number of other research resources in reaching your decisions.
Many students (parents) get so hung up on the rankings that they aren't open to investigating really great schools that didn't make it to the top of the ranking list - institutions where the student could get a wonderful education. That's quite a shame and is a great disservice to you - the student. You've probably heard it many times before, but remember that what you're looking for will be the best "fit" for you, and that's something so individual that it can't possibly be part of a ranking system.