I've thought about getting one of these a few times. To me, they just sound like tendonitis calling. They might do more harm than good. If you really want something in your hand, I recall seeing a product that fits in your palm that has tensioned strings and about 4 frets to press down on. I can't find the link but something like that would be a better bet, IMO.
And for what it's worth, I feel that some of my most important practicing is done without a bass. All day long while I'm listening to music on the radio or in my head (the voices are nice

), I'm shedding along with my fingers, running through scales and modes relative to the song, etc. Then, later, when I pick up a bass, I find that my dexterity has improved and my fingers flow faster and more smoothly. You may be getting more out of your air bass than you think.
