I'm in much the same boat as you E. I got into both the books, and movies, a little later than most. We didn't see the first movie in theaters, and I think I started reading the books right after Goblet of Fire was released in theaters. Like you mentioned, I saw them all on DVD - and really enjoyed them - but for some reason just didn't get to them in the theater. I often feel like I missed out as well, both on the movies and book side. I'm sure the hype for both was a lot of fun to take part in.
Since then, I've read the whole series a couple of times and I'm actually going through them again right now for the first time in awhile (currently on Prisoner of Azkaban). I also enjoy the movies and books nearly equally, and think all of them are very well done. I think the books are probably my favorite reads of all time, and the movies are up there among my favorites as well. I've said before, this universe often reminds me a lot of Star Wars when I was a kid. The movies....at least for the most part (as they've gotten darker)....are something the entire family can enjoy together, with a heroic trio (and supporting characters), a central villain, cool creatures/effects/etc., and a great good vs. evil story. Like you already mentioned, I got into this series as an adult as well, but I bet it is a lot of fun for those kids who truly grew up with this series.