They also have problems with waking from sleep and staying mounted sometimes.Performance and reliability are the two main concerns.
Performance of the sdcard is much much less than the internal SSD. I used mine for running low activity VMs where performance didn't matter to me.
An sdcard also doesn't have the same level of garbage collection, wear leveling, or trim support that a proper ssd has so the chance of failure could be higher. I wouldn't rely on one to store anything important that isn't backed up.
Ditto wondering the same thing, I have a 64gb card laying around and contemplating the nifty for non-critical storage and convenience, trying to determined if its worth the $40I'm reviving an old thread here but what are people's experience with Nifty's sleep/wake issues? I'm debating getting one for my new rMBP to keep my iTunes library on. I already have an external SSD but I'd like to keep some things with me for convenience (and not juggling my four external drives).