It was quick and easy - and yes, I'm willing to pay for convenience, so it was totally worth the uncharge. (Although, honestly, I'm not certain they actually charged us! I'd have to look back on my folio for the stay. But, assuming that we did pay the $6 fee, yes, I found it totally worth it to not have to lug a case of water all the way across the Pop grounds!). Each of us threw 2-3 bottles in our backpack each morning, and though there's definitely some weight associated with that, none of us found it to be problematic in the grand scheme of things.
I absolutely agree with other posters about not wanting to use single-use plastic... we actually purchased awesome collapsible silicone water bottles with carabiners specifically for this trip, and we used them on our first park day... and seriously, the taste of WDW water is SO bad that none of us were able to stomach it. When the choice was between staying hydrated and using single-use plastics for a few days, we chose staying hydrated.
I appreciate the suggestion of bringing flavor packets, and I'll maybe try that next time.