They were my favourite books growing up, but to be honest, now that I am something of a children's literature buff, I realize I didn't read very good books back then. It was just whatever my parents bought at yard sales (Goosebumps, Boxcar Children, Archie Comics, etc.). They're not the best children's books ever, but they are a good combination of mass appeal and quality. It turned book releases into major events. And people love being a part of big cultural events, fandoms, etc.
IMO, whenever there is a big following/fandom surrounding a show, it becomes a bit insufferable to people who aren't big fans (I like Sherlock, but the obsessive people on Tumblr irritate me). So naturally, there are going to be a lot of people who find the whole Harry Potter phenomenon to be overblown. They're not wrong.