I tend to read a lot from other forums and some guys mentioned that under hot weather or extreme use it produces error, and images lost in the process. I think one guy here face the problem too. It might be cheaper than CF card, like Hitachi's MicroDrive (formerly from IBM) but the cost advantage is greatly reduced to nothing with the building of more fabs by AMD/Intel/Samsung etc for production of more flash memory. Imagine AMD is earning money from flash memory than from CPU business can tell that the market is there.
The other great reason for not getting this disk based cards is that we have the better built, more functional portable digital wallets like x drive, archos, tripper image tank and super digibin etc etc, which typically costs from $400 - $900, affordable when you can spend $800 for 2x 2.2GB MD while a 2x 256MB CF + $500 Digital Wallet is about $750, and is 20-40GB of storage.
Just my opinions to share