We're working out what we think - we'll add our view here!
Hydrapak says:
Quickly connect a drink tube to your HydraPak Seeker or HydraPak water bottle for versatile backcountry use with the Plug-N-Play™ cap. Use inside your pack like a hydration reservoir, strap outside like a bottle, or quickly setup a gravity feed at camp.
We're working out what we think about Hydrapak's Plug-N-Play Cap, we'll add our comments here in due course.