The Nifty Drive is a Micro SD card adapter designed to sit flush with the unibody of a current-generation MacBook. It's a Kickstarter success story, but not one without some bumps in the road. The Nifty Drives currently available for purchase via theniftyminidrive.com are the third generation of adapters, the previous two were plagued with different design and manufacturing flaws.
The drive is a great idea; where a standard SD card is larger than the slot and sticks out to enable removal, Micro SD obviously are not. The Nifty Drive has an aluminium cap on the end that should blend in to your MacBook's unibody (or stand out, if you choose a coloured one). I found that it was possible to insert the Nifty Drive a few millimetres too deep on my retina MacBook Pro, and it took a small amount of patience to get it to sit flush. Once situated however the drive looks great.
The fit for the Micro SD cards is tight, and customers who bought Micro SDs that innocently came with sticky labels attached would get stuck in the slot. If these sticky labels are simply removed the issue is avoided. The space constraints in the design are unforgiving and the guys have done a really great job of it. I need to purchase a larger Micro SD to plug into mine, but the 2Gb one that came with the Nifty Drive is working perfectly.
One of the benefits of SD cards is their resilience. They're able to withstand conditions that would obliterate most electronics: water, pressure, heat, electromagnetic radiation. They obviously have their limits but the data stored on them will certainly survive many accidents that would total the Hard Disk or SSD in your MacBook. This makes them a great candidate for storing important data, or for a local backup of your data (you should still maintain remote backups). Data backed up to the SD card can hopefully be retrieved if your machine is irreperably damaged.
The other benefit of course is simply expanded storage. The move to SSDs in MacBooks has constrained the internal storage somewhat in favour of the lightning fast data transfer speeds that flash storage offers. In theory an SD card could hold terabytes of data; in practice cards are on the market for up to 128Gb of storage. Some MacBook Airs ship with less storage than this. It's not as fast as your SSD but not all your data requires that kind of speed. It's a great way to expand your available space and is upgradeable as bigger SD cards become available.
I'm really pleased with it.