Long time reader, first time commenter. As someone who backed this Kickstarter, and considering some people are mystified as to why you would want this, I thought I might be able to provide some insight:
As an owner of a 128 GB Macbook Air, space is at a serious premium. An extra 32 GB or 64 GB would go a long way. I'm also on the go a lot, taking my laptop to and from work, using it on my commute, etc. Now, existing USB memory sticks, however little they protrude from the port, are always going to be a concern: they may fall out, or get snapped off in my bag, not to mention they use up a valuable USB port (I only have 2 after all). A miniSD-to-SD adaptor also isn't suitable: like an SD card, the adaptor would also stick out, making it likely to snap off or come out in my bag if I leave it in all the time, or for me to just lose something that small if I take it out all the time. This is especially an issue when, for example, I reach my stop while I'm in the middle of work and just have to close my laptop and put it in my bag without having to wait to eject a disk.
This product is clearly not for people with integrated miniSD card readers in their laptops (which is great for those who have them), and it's also not for people who leave their laptops at their desk all the time and can just use an external drive, but for someone like me this is perfect. I get the extra space, it doesn't use up any valuable ports (I rarely use the SD card slot), and it's not going to get lost or broken.
Possible uses for this extra space: (for me personally) I'm going to redirect my Downloads folder to the drive, and also use it to as a temporary backup when I'm on the go.
Just my two cents. £20 is a steal. Some people may think this is too expensive, but the laptop cost me £1000 when I bought it, and this £20 allows me to permanently add an extra 50% hard drive space without having to do something like void my warranty and swap out the flash memory. What's my alternative? Buy a new laptop? No thanks.