I was travelling a couple of years ago with a similar product that was NOT waterproof, and found that in particularly hot countries, documents absorbed any sweat (sorry...) that occurred during the day. This product completely prevents this, as the plastic wallet folds over a couple of times before the velcro strip seals its 2 pockets. I wore my every day for 2 months this year in very hot and humid conditions and it only has a little bit of stretch in the elastic band (which is also very comfortable).
The clip on the right side of the pouch is also very easily accessed and opened/closed, and I never had problems with it opening when worn - ie. it was always secure.
The only downside is that it's a bit obvious under clothing, and can be a little bit awkward to access if hidden, but this is a problem I think comes with having any hidden wallet! Well worth it to keep your valuables in - passport, tickets, money (notes) and credit cards can all be held in this comfortably at one time.