Warning: This whole thread is only about historical information now since it's 2018 and we can buy inexpensive and great performing A1 rated SD cards in the meantime. Buying anything else is a mistake so directly jump to the end of the thread for performance numbers and recommendations.
Edit: See an early 2017 update at the end of the thread regarding new SD specs also covering random IO performance.
Edit 2: Some thoughts/observations on lifespan/reliability
Some quick test with f3write on eMMC on Orange Pi+ 2E. Writing speed starts with excellent 24MB/s. After approx. 3GB written data the writing rate drops nearly instantly to 7,5 MB/s and stays for it until the end of the 12GB.
Any clue why this happens? Reading rate is about 60MB/s, not so bad.