I actually think the best approach, longterm, would be to use the SSD as a cache for the HD at a filesystem level. I believe something along those lines was just merged into the Linux kernel.

Yep, dm-cache essentially does what Intel Smart Response does except it's done in the OS and not the BIOS.

If the SSD part fails you pretty much lost all benefit of the HDD itself. It might or might not function with the bad flash chips but you definitely lost all the performance boost. The caching options sound much better overall but they do take more space (2 slots vs. 1 for the hybrid).

With hybrid I didn't mean those hybrid HDs but two separate storage devices using two slots.