Xiaomi has been quite successful in China with its flagship "Mi" series, and the recent mid-range 4.7" Hongmi device, to the point where it's now the 5th most popular smartphone brand there. They are now getting ready to launch the Hongmi 2, but this time with a 5.5" display and an 8-core Cortex A7 processor (instead of quad-core as before).
I still don't see the point of true 8-core chips this early in the industry, though, and right now it's nothing more than a marketing gimmick. I'd rather see OEM's try to push chips based on the 64-bit ARMv8 architecture first, and making them at 20nm, and then at 14/16nm FinFET, before moving on to making 8-core chips. Those kinds of improvements would be a lot more beneficial and more practical for users today, but also in the future, while an 8-core chip will only be useful in the future.
The Exynos 5 Octa at least had more practical applications today, because it was really just a cluster of 2 quad-core CPU's rather than a true 8-core chip, utilizing whichever cluster was more appropriate for the task at hand.
I also think seeing more co-processors like the ones Motorola's X8 chip has, or Tegra 4 for HDR, will be a lot more beneficial in the short-term, especially for stuff like photography. The more processing power we throw at computational photography, the better the smartphone cameras will get, even if they will never be able to use sensors that are as large as the ones in mirrorless cameras or DSLR's.
But that doesn't mean I believe 8-core chips will never be useful. Their time will come, especially as chips start supporting OpenCL 2.0 for parallel computing, and are built according to AMD/ARM's Heterogenous Systems Architecture. Until all of that is supported, 8-c0re, 16-core, or 32-core chips that will inevitably arrive by the end of the decade, will be nothing but marketing gimmicks. But even if they do arrive, the app development for these kind of chips will still lag behind the hardware, because developers will only start developing apps that are optimized for these architectures, once there is a huge installed base of such hardware to begin with.