I started using the liners in my 6gal carboys too. I have not used buckets since I went all-grain, but it is tempting for beers with lots of whole hops. I ordered another box of bags that are 3 inches longer to leave more hanging out of the carboy, making it easier to work with. I like that I can fill the liner with wort or beer then fold down the excess plastic to keep dust out before I am finished (fermcap, aerate, pre-fermentation additions, pitch yeast, airlock or blowoff). I found the metal lid ring from a non-wide mouth canning jar fits over the neck/bag easily to keep the bag folded shut.
S-13572 12-16GAL 1.5MIL CLR TR
For a while I tried going without the liners, but I was never completely pleased with how clean my carboys were coming out and spending a lot of time scrutinizing and re-cleaning. I went back to using the bags because I can pull it out in a minute, rinse the carboy with water and call it done.
I've found the liner makes it easy to remove a #10 bung which you can sink to be just below flush with the top of the carboy. Fits nicely, and because you can tug on the plastic to pull the bung out, you can do it with one hand. This makes things easier when getting ready to rack since I can be holding my racking cane in the other hand.
The backpressure outside of the bags but inside the carboy was reducing my headspace so I actually drilled a hole in the shoulders of my plastic carboys, allowing the bag to fully inflate when the yeast get going. This has added advantages, for example I don't have a suckback problem because the bag can deflate, and if I remove the bung to take a sample, I can blow air into the shoulder hole to deflate the bag, pushing out a good portion of air that may have entered when I took a sample. The shoulder hole also makes it easier to remove the bag because I can use backpressure to push the bag out. Alternatively, pierce the bottom of the bag with something long and sharp like a racking cane.
I've been using the bags for lots of stuff, like containing spills if a bottle broke while shipping it, lining my grain buckets, and even as a trash bag in my brewery