Like szts77 says, I began with the books as well. Once I saw in the trades that the first movie was well into production, I could tell it was going to something big. The books were already huge and changing the ways kids were even thinking about reading, so being a junkie for all things pop culture I went out and bought all the Potter books that were out by that point. It only took me about a week to read the first four, so the time commitment isn't really high.
As for why you should read them, i'm not going to jump in the deep end of the Potter fanatics and say J.K. Rowling is one of the best authors i've ever read, but there is something simple, pure and reminiscent of childhood fantasy in this series. Harry Potter is a charmingly flawed character that you instantly feel the need to route for (even after you realize he gets himself into more trouble than anyone else and usually needs his friend to bail him out).
For me, charming would be the best way to describe the books. Whenever I finished one of them, I always had a soft smile on my face and I felt my time (what little of it I needed to actually read it) was not badly spent.