Yet, I can't help but think that while this device might be psychologically usable, it probably isn't very ergonomically useful. That is, it does a good job of cleverly conveying information, but the fact that its size increases might make it uncomfortable to carry. My pockets are already too full with a simple key chain and a regular USB drive (well, I actually have 2 USB drives)--I'm not sure I want to carry something around that, as the day progresses, will get bulkier.
What's more, while I applaud the design for the slickness with which it informs the user through a channel not currently exploited, I don't think the information represented by its changing size is all that useful. Unless I have to pick out a USB drive from a pile of five or six similar devices, I don't really need to know how full the drive is. And, once I plug it into a computer, which I'll inevitably do, I'll know just how full it is then. I guess one situation where the inflated state might come in handy is if I'm in a rush in the morning and know that sometime later that day, I'll need to put stuff on my USB drive. If I pick up the drive and see/feel that it's fairly full, I'll know I have to take a couple of minutes and empty the current contents of the drive on my computer.
But still, after I fill the drive up with data, it's going to be big and bulky.
I think a more effective (although admittedly less playful) design would be to add a decibel-like meter, a la the wrist meter used by Tom Cruise in Mission Impossible, sort of like the image to the right.
Or even cooler, an classic analog needle shrunk down to the size of a USB drive.
I heard about this device here. The images are from here: image 1, image 2, and image 3.