Slow write speed,
This review is from: Komputerbay 32GB MicroSD SDHC Microsdhc Class 6 with Micro SD Adapter and High Speed USB Reader (Personal Computers)
I bought a class 4 Toshiba - Flash memory card - 16 GB - microSDHC about a year ago.
This (supposedly) class 6 card takes about 25% longer than the old class 4 to write a single 100Mb file (about 25s).
Read speed is nominally the same as the class 4. (All tests done through the adapter to a SD reader slot on a PC)
Having said that, this is OK as simple storage expansion for a phone (once you've waited to fill it up). I wouldn't get one for a fast camera.
The price is pretty fair considering you get the adaptors as well.
4* for value.
2* for performance (if it's important to you)