Overrated? I think that depends on the context or what scale it's being measured on. As "great literature"? Probably overrated. As "a fun, engaging story"? Probably gets the credit it deserves (which is a lot -- for me, anyway). As "actually got 90's-00's kids to read"? Might not get as much credit as it really deserves. I was a kid in the 80's, and the only comparable cultural equivalent was Star Wars. Now, I love Star Wars (I actually mostly tolerate the gruesome prequels, even), but there's a big difference in getting kids excited about movies and toys compared to getting them interested in books -- especially when 90's-00's kids had so many other options, things like video games that weren't around in anywhere near the same way when I was a kid. Big props to JKR for that, even though I'm sure the books' success was beyond her wildest dreams and more than she intended.
I don't care so much about the movies. They're enjoyable, I've watched them all in the theater or on DVD, and I'll probably get the last book (two movies) adaptation on DVD when they're both released. But I don't really get all of the movie-hype. I already know how the story ends, which was the big draw.