Yeah the number one spec (that the manufacturers will never list) is the maximum latency for writes. That's what causes dropouts, because the OpenLog's buffers overflow with data while it's waiting for the MicroSD card to complete a write.
I found a benchmark here where several cards were tested:
http://forum.arduino.cc/index.php?topic=258667.0
Cards that look to perform well from that test:
SanDisk 16GB SDHC Extreme (~10.5ms max latency)
SanDisk 32GB SDHC Extreme (~10.6ms max latency)
SanDisk 8GB SDHC Class 4 (~18ms max latency)
SanDisk 16GB SDHC Ultra (~20ms max latency)
Samsung Pro 32GB (~23ms max latency)
Samsung Evo 16GB (~30ms max latency)
Samsung Evo 32GB (~30ms max latency)
Cards which look to perform poorly:
Samsung Pro 16 GB (~37ms)
SanDisk 32GB SDHC Ultra (~70ms)
Samsung 8GB Class 6 (~104ms)
EDIT: So it turns out that the benchmark tool used on those cards in that thread is the one that ships with the SdFat library. That's the one that the OpenLog firmware uses. So if you've got that library installed, in Arduino IDE you can just click File->Examples->SdFat->bench, then compile and upload that. If you're programming the OpenLog with an FTDI cable, just click Tools -> Serial Monitor to show the output from the serial port. It should ask you to press any key and then it'll begin the benchmark, printing the results to the screen.
Normally I just delete my logs off the SD card after recording them, but this time I gave it a full format with the sdFormatter tool. The difference was pretty immense, the maximum write latency is now about 1/10th the size of what it was originally, which is a massive performance boost! (Dropped from about 95ms in some tests to 10.6ms most of the time).
I'll post more test results to that thread as I dig up cards from around the house!