I use the slot all the time, but only for putting in things like my camera SD card to copy off pictures/files. But I guess there's no reason not to use it to add some storage... I think the biggest you can get is 128gb though? and it's pretty expensive,
Apparently the SD Card reader on an imac, is attached to a really fast bus, not just USB 3.0. Not sure that it matters, given that SDCards aren't known for their speed. I use mine to transfer photos from my Nikon.
I have several "half" cards that I use in the SD card slot. I have several brands, as well. PNY makes the StoreEDGE cards in multiple storage sizes (64GB and 128GB). The other brands - Nifty and TheMiniDrive use MicroSD cards, and the ones that I have currently support up to 128GB MicroSD cards.
I use them for storing work projects - photos that I am working on, documents related to other projects, and one that I use to store movies. The StorEDGE has a small lip that sticks out just enough to allow you to grab it to eject the drive. The other versions sit completely flush with the SD card slot, and have little slots where you can "hook" them with the provided tool to eject them.
I use my SD slot as a quick jumper between all my devices: PC, macbook air and iMac. I named it Apollo and it stores all of my important documents that I would need if mega failure happened to both my internal and external HDD.
Also, I am looking into getting one of the 'half' SD cards for my macbook air to increase its storage capacity but still keep form.