Personally, I've tried Kingston and Sandisk MicroSDs (Euro and Japanese versions) and haven't noticed any difference between them. They were all class 4 memory though. For those who don't know, "class" is the performance rating, so the higher the better. My personal rule of thumb is always go for class 4 or better as it is a good compromise for cost and performance.
Most MicroSD cards tend to be class 2 in Europe and the US, but I suspect Japanese versions tend to be class 4 hence their reputation.
I've heard Toshiba's class 4 are very good if you can get hold of them, a slightly better sustained through put than other manufacturer's class 4s.
Krell