Very cool. And it's about time something is seriously done to penetrate the very imminent performance limitation of current memory technology. For even though there are still an available roadmap toward DDR5 (DDR4 will probably just as in graphics be leaped) and XDR2, the limits of these technologies seem to peak out at about 4x current DDR3 performance. So if HMC starts out at 15x the current DDR3 performance, and a likely HMC2 upgrade later on the memory roadmap is secured for the next 4-5 years. After that, who knows, probably memristor technology.
I see this more for the high end server computing. Mainstream desktop computing will start integrating the main memory inside the CPU as a Level 4 cache. This is already done in the cell phone and tablet market, where ARM CPUs have 512MB/1GB of memory embedded with the CPU/GPU. A Trinity APU + 4GB embedded DRAM would satisfy a great majority of the market. You get huge power savings by not having to go off chip for memory.