I resisted buying an inflatable sander for the longest time, primarily because of price. However, I found that the flexi-pad sander was really not suited for sanding inside curves, and I wasn't about to fabricate my own sanding ball. I've never regretted my decision.
To get started, the only things needed are the ball, a small hand pump, and a set of sleeves of assorted grits. The ball chucks into a drill press, which leaves your hands free to hold your project. As I started working with square and rectangular shapes, I found the small drum useful for getting into corners.
And, once you have these tools, you'll find all sorts of uses for them. The adjustable firmness gives a degree of control that you just don't get with rigid spindles.