I have a mix-and-match of the paper books: first two books in US paperback, three and four in UK paperback - all four I got used from a friend who was replacing his entire set with UK hardbacks; five and seven bought as brand new (pre-ordered) UK hardbacks (children's edition cover), six bought as brand new (pre-ordered) UK hardback (adult edition cover).
I also have the first four books in Estonian (bought brand new hardbacks) and the first book in German (bought brand new paperback).
I've had darknet versions of the ebooks for years (being actively part of the HP fandom, there was often a need to check something in the books and search worked a lot better in PDF copies than in paper books, oddly enough) although I never
read them; they were strictly for research purposes.
Now that I've got the legal ebooks as well (bought the complete set on Pottermore) I might actually re-read them as ebooks.
ETA: I also have Quidditch through the Ages and Fantastic Beasts, both in Estonian, and I bought the Amazon exclusive limited edition of Beedle the Bard (think it was 99 euros or something like that) when that came out.