Round-ended sanders are absolutely necessary for sanding the inside of scrolled bowls with curved sides, ripples, or petals. Before the availability of the Guinevere system from King Arthur's tools, anyone wanting a tool of this type had to construct it themselves.
The specific sanders from King Arthur's Tools that I use constantly are the large and small round inflatable sanders. The rubber "ball" inflates with a small pump, and uses sleeves of various grits. Although the sanders can be fidgety at times, they are extremely effective. In addition, repairs can be made to the inflation mechanism, and the rubber ball itself can be replaced if necessary. The large sander costs about $44, small about $40, sleeves are about $3.00 each, and the pump is about $11.
I was not aware of any other product that had a rounded configuration until I came across one offered by Seyco, called a "rounded end cup sander". The configuration is similar to the KA product, but it is made of soft foam covered by a non-removable sandpaper sleeve, attached at the top with tape. It is available in four different grits, and costs about $7. Curious, I bought two of the coarsest grits at the Fox Chapel Open House in May, and just got around to testing them out.
To start with my conclusion, if you are serious about bowl sanding, go with the King Arthur product. Although the initial outlay is greater, the $7 replacement cost of the Seyco product will quickly add up. A set of four sanders, one of each grit, costs $28. A set of assorted sleeves from KA for the large round sanders costs $11.
In terms of performance, even the coarsest grit from Seyco was not effective for that all-important initial sanding. In addition, the stem, which is glued to the foam, quickly came unglued. I did like the size, which was intermediate, but found the longer configuration not as easy to use as the more ball-like KA product. The fact that it's ready to use is handy, but you pay for it in an inferior performance and greater ultimate price. And, for many projects, you absolutely need the small size of the KA product the get into those tight lower curves.
If you're curious about this type of sander, there's no harm in buying one or two Seyco sanders to test them out. And if you already have the KA system, you may find the medium size of the Seyco product handy at times.
However, as most of us quickly learn, there's no substitute for a quality tool!