I bought these cards to use with my Raspberry Pi computer, for which Class 4 or 6 is recommended due to an issue with Class 10 cards. I thought 4MB/sec would be sufficient, so I was quite surprised when I popped them into my cheap Akasa card reader and found they easily managed sustained write speeds of 12.5MB/sec, and read speeds nearer 20MB/sec, far higher than you might expect of a Class 4 card. I had no hesitation in recommending a couple to a friend who needed something fast for his new camcorder, and sure enough they're more than quick enough for recording 1080p HD video. Faster than Class 10 and a lot cheaper too - you can't go wrong.