I hadn't bought an SD cards in a long time, but picked up my first SD card only camera I've owned in a while not too long ago(all of my others are either CF or CF+SD), a Nikon D600.
I initially made the mistake of putting Class 4 cards in it, and with two class 4 cards I ran into serious issues with filling the buffer when shooting a sustained ~1 frame per second. That's JPEG fine + RAW, with each file type going to a separate card.
I bought a pair of Class 10 Sandisk 64gb cards(Extreme Pro) and the camera gladly keeps up with my shooting speed.
With that said, I've never seen a camera reduce its quality setting to match the card speed.
All of that aside, though, I'm VERY particular about where I buy storage media. Counterfeit CF cards aren't really an issue anymore, but they are common on SD cards. Counterfeit cards often don't live up to their speed claim or don't have the data integrity of actual high end cards from reputable makers. I pretty much only buy from B&H photo, although I would trust Adorama equally.