The problem is that it's touted as a great way to scale web apps. Since DB performance is usually the limiting factor in webapp scaling, this doesn't appear to stack up. Needing a lot of IOPS is pretty par for the course.
I guess my question is why go for reasonable when you can get incredible performance per dollar with RAID-10 SSDs? A few dedicated DB machines with SSDs and many cores can get you absolutely monstrous throughput without going for any of the more exotic DB solutions.