We just got back from a 12 day trip and bought the bucket on day #1. We are a family of 4 and my 2 kids love popcorn. We got popcorn on most of the days and here are some of my/our observations:
Except for the maple, the three other available flavors were just okay; tasted like something you buy in the bag at the grocery store (I'm guessing is that Disney gets these pre-made as well or pre-makes them). The buffalo blue cheese smells horrible! The flavoring for all the flavored popcorn is like the stuff on cheetos and "stains" your fingers. Although probably doesn't make a difference, just to see my kids licking their fingers after eating this stuff made me shake (even though we have them clean their hands with wipes before eating the popcorn)!
The regular popcorn and maple are good, but when you get them fresh and hot out of the popper, they're really good. We could eat that stuff 24/7!
We would normally get one refill during the day, and on a few of the days, one as we were leaving the parks to take back to the room for later. Maybe it was because of the humidity, but the saved popcorn was never any good; after a few bites we would always dump it.
We tried to use the bucket in HS at Fantasmic and they don't refill your bucket, but instead, give you a box of popcorn just like what you would regularly buy. It wasn't good and I thought I was eating a box full of those packaging "peanuts."
Also, for some reason (and I could be making this up!) it seemed like Disney popcorn gets "stuck" in my teeth more than movie or made-at-home popcorn.
If you are on any of the dining plans, you can use one of your snack credits to cover the refill, but obviously not a good value (also make sure that you tell the CM you want to pay for it, or you'll most likely get charged a snack credit).
As for carrying the bucket, I had brought with us a carabiner clip that I either attached to our backpack or just to one of my short-belt loops on the days we went backpack-less. I didn't find it to be a problem either way; it's not heavy and on the smaller side to carry. After a few days, my family started making fun me because I was carrying the thing like it was my baby!
Although many of the above comments seem negative, overall, I think the refillable bucket is a great deal, wether you like the flavored or regular popcorn.