With .epub, some files seem to use a different "standard font size" and as search, you will have to change the font size from book to book. Or you can change the font size using a program: such as Calibre before loading them Onto your ereader. Other than that .epub files are handled great, with quick page refreshes it's a great reading experience.
With pdf's it is a little different. The overall experience is very good I must say, but with pdf files thathave Large resolution photos, the H2O will struggle. Page switching will be slow and loading the pdf will be frustrating.
Another minor issue is indeed some textiles, if you are wearing long sleeves or reading in bed, wants to activate the "water droplet" notifications, Which can get annoying. But again, this is a minor hangup.
Another issue did hopefully will be fixed in the near-future is the factthat you can not organize your books / files in folders. Instead you have one long list, and without a search function it can become tedious if you have a lot of files to keep in order.
I hope this will be fixed with at upcoming firmware update!
Overall I'm very satisfied, but be aware of the minor issues did come with it to avoid unnecessary frustrations.