Nifty gadgets, could be fun, but I'm not sure they're worth it. I guess if you are doing a barley wine or something, they might tell you if you've got a stuck fermentation, but otherwise once primary fermentation is over, you're pretty much assured of being in the 1.01Xs somewhere, and only gonna drop another .002 or so in secondary (if you're doing secondary).
I do all my primary fermentation in a bottling bucket, so taking readings is as simple as opening the valve. It also makes racking to secondary easier, although my 2nd is a better bottle, so I do still have to siphon to bottle if I go that route.
I wonder if the balls would even work at high krauzen. I would think they would just ride on top of the foam.