I see a lot of praises for class 6, class 10 in this thread
I would like to share my own experience, I have two 8gb Sandisk Class 2 micro SD card that I use in my K-r with SD card adapter. It wont be fair comparing a Class 2 Sandisk with Class 2 Transcend :P because Sandisk Class 2 outperforms Transcends Class 6 in terms of write speed.
Class 2, Class 4, Class 6 and Class 10, all represents write speed i.e. Class 2 cards will have minimum write speed of 2mbps where as class 10 will have minimum write speed of 10mbps in fragmented state.
I performed a test on formatted Sandisk Class 2 SD card and found maximum write speed of 9mbps (its was formatted so no fragmentation, it was sequential writing)
I never found any issue while shooting vidoes too, I took 3-4 mins clips and file size where whooping 600-700megs. So looks like a minimum write speed of 4-5mbps will be good enough for HD recording.
I am very happy with my Sandisk Class 2 cards but that doesnt mean you should go buy a class 2 card
I would recommend you to buy a Sandisk card with Class 4-6 (that will not burn your pocker) rather than going for Class 10 Transcend
Also, just format your card before you start your trip, its a best way to get max write speed