This SSD certainly hits a nice pricepoint cramming in 4TB of SSD space in a single drive for only $500 USD, but the idea of it isn't as appealing to me as would be a drive with a faster throughput spec than just SATA III. An SSD of this size definitely needs SATA Express compatibility and speeds, as well as an alternative of just slapping two different PCB's together using JBOD on a unified PCB - this is the point where you have to wonder about the reliability ratings of each individual side of the PCB since it's dual drives in a single enclosure. The option of perhaps reducing the SSD storage to 2TB and giving you the hardware option of RAID 1 would be something completely different though, and would definitely give this drive an advantage over other SSD options on the market right now. I would expect Mushkin to come up with a solution that could unify all of the NAND into a single bank of chips and increase IOPS all around (that is if they can come up with a good quality controller chipset).
If things came down to it though, I would probably be more intersted in dual 2TB SATA III SSDs versus a single 4TB drive in that configuration since you'd end up with the same reliability ratings as well as benchmarks. The term "never put all of your eggs in one basket" comes to mind. Of course, this is just all personal opinions and could definitely be a worthy contender in the SSD market especially for people who need that kind of storage quickly such as video production and such. Mushkin tends to be a pretty well-liked company with solid hardware though, and I actually use a Mushkin ECO2 SSD in this very laptop I'm using right now and it gets stellar benchmarks and has given me no problems at all.
In the end, I can see a few areas where buying 4TB of SSD storage at this price point could definitely be advantageous to end users....but not all SSD users.