I am nineteen, which in term of Harry Potter means I began watching the series when I was really young. So there was never a time "before" the series, a point at which I could have decided to read the series before watching the movies - because lets be honest... its always best to read the books first- and this mistake haunts me to this day.
I've tried to read the books now, but the characters are hard to imagine when I know how the directors cast them, and the changes are hard to stomach when I know they didn't "happen". Nothing feels right when the imagination aspect of reading is stollen.
The hardest part isn't that I am unable to struggle through, its that everyone else has. They all know these inside secrets only the book readers know, and they all comprehend the characters in ways I can't... but worst of all they all don't get why I can't understand like they do.
They think I can just pick up the book and see what they see, but all I see are the movies and the characters I already "met".
Thats the problem with watching the movies first, I'll never be able to imagine my own Harry... it will always be the movie version, Dumbledore will always be too aggressive and forthright... my perspective will never be original just eternally skewed.
If you want my opinion, don't ever make my mistake. Don't watch the movie first, read the book because its always worth it.
(The image is not mine, but was found using google)