Update 2010/01/21: Thanks to @ceolaf for pointing out that Amazon has updated their page. They accepted my correction: "Fast read speeds of up to 15MB/second; write speed lower".

I have been using a Transcend 16gb Class 6 SDHC card Transcend 16gb Class 6 SDHC card in my Canon T1i, but needed another card. I found a good price on the somewhat confusingly labeled Sandisk Ultra 16gb SDHC card Sandisk Ultra 16gb SDHC card. As you can see, it prominently specifies "15MB/s*" on the label, and on Amazon's page. Additionally, it shows a smaller "C4" (Class 4) logo.

SDHC Class ratings are minimum write speeds in mbytes/sec, so the Transcend is guaranteed to write files at 6mbyte/sec or better, which is Canon's recommended minimum for shooting video or rapid stills on the T1i. I was a bit confused to see the C4 logo on the card, but I believed Amazon's "High speed card featuring fast 15MB/sec Read/Write speeds", which would be effectively Class 15. I couldn't find any explanation for that asterisk, but the price was decent, so I ordered a card.

I just got the card and did some simple tests -- copying a 1.1gb file to and from the card through my USB SDHC reader -- and discovered a few interesting things. The SanDisk wrote at 10,600,679 bits/sec and read at 21,621,758. The Transcend wrote at 9,399,092 bits/sec and read at 18,514,338 bits/sec. This means my SanDisk Ultra card qualifies as Class 10, and my Transcend is almost as fast (but there is no Class 7-9, so Class 6 is correct). The SanDisk is much faster than its marked Class 4, but nowhere near the 15mbyte/sec write speed Amazon promised.

Tonight I did some more poking, and found http://www.sandisk.com/products/imaging/sandisk-ultra-sdhc?tab=features, which explains that asterisk:

Fast read speeds of up to 15MB/second*; write speed lower

So Amazon's page is wrong, and I sent them a correction. I'm keeping the Ultra card because it's fast enough for the camera and probably whatever else I'll use it for, but disappointed it doesn't deliver on Amazon's specs.