There is raging debate about USNWR's ranking of law schools. A bunch of ABA deans signed this letter disapproving of how USNWR conducts the survey as it hurts more than helps. Basically, if you want to get into Big Law (aka big law firm where they sweet talk you that first summer associate year and then put your neck on the grinder for 80-100 hrs a week), then top tier schools are a must. Whether you like it or not, employers look at the rankings, however flawed. Students look at these rankings when choosing schools, whether such schools are actually the best "fit" for them or not.
Actually, the USNWR's rankings are based part in numbers and part in reputation. Therefore, whatever school that spends the most money on mailers, brochures, whatever will generally have a higher ranking. There was another study that showed the halo effect of schoolon rankings. The survey revealed that many thought that Princeton Law School was one of the best. They actually don't have a law school.
It all comes down to what you want to do and where the jobs are at. If you just want to work in a small firm or med. sized firm, then you can go to a lesser tiered school and still get a great job.
Added: if you really want to become a lawyer, I suggest you take some time and work in a law office. I've read a bunch of books one lawyers getting out of the legal field b/c they are so sick of it. The rate now is around 50%. The practice of law isn't the same as going to law school, and for some, the work is pretty boring.
Just my 0.02.