Because I use carboys and sanke kegs for fermenting, and I also keg, I purchased the Large Carboy & Sanke Keg Cleaner, and then purchased the extra pads for the smaller carboys and kegs. It works very well.
Yes, you can use a 6.5 gallon solution of warm PBW and soak for a day or so, but I'm generally cleaning several carboys and kegs on the same day (save up for a brew day while I'm sitting around), which makes something like the Carboy Cleaner much better for my purposes. Another advantage is that you use less PBW (only need to mix up half a gallon or so worth).