Maybe mine is defective, but I have an Armpocket 20 that lets water in.
The first time, I ran about an hour and 20 minutes most of which was in the pouring rain (rained more than 1/2 inch during that time). I normally would not have headed out in such a downpour, but I looked at the radar and all seemed fine as things were moving away from my location and I had the new "water resistant" Armpocket, but rain blew up after I left. When I got back, I tried to wipe the drops off the outside of the plastic so I could see and oops, they were on the inside. The water sensors are all still fine though, but the phone was wet back and front.
The second time, it was more of a light mist and while the water infiltration was much less, there was still condensation inside the Armpocket.
I have run both times with the zipper down and headphone port up, which seemed like a pretty good seal, but I may try to silicone the headphone port as I use a Jaybird SB2 bluetooth headphone, so no need for an extra hole.