Just wondering: Is there a point (or is this close to it) where in using HDDs and certain RAID configurations, you can match or beat speed while maintaining better redundancy with larger capacity, cheaper drives? What is the main application these excel at? I assume power would be one, and cached content on webservers? Help me understand :-)
You'd need several dozen hard drives to even approach the IOPS of a single consumer level SSD. The SSD wins so many times over it's not funny.
Now, if you're talking about sequential read/write speeds, that's a whole different matter. You'd need roughly 3-4 hard drives (in RAID 0 (no redundancy)... double that figure for RAID 10) to match the typical sequential read/write speeds of an SSD. At that point, the raw cost of the hard drives far exceeds that of the SSD, and that's ignoring the need for the extra SATA ports, cooling, physical space and the extra drive failures you need to deal with. So, the SSD wins again, hands down.
Now, say you needed to store more than roughly 200 gigabytes of data and performance didn't matter at all, in that case, hard drive(s) will be more cost effective than SSDs.
Basically, hard drives excel at bulk storage of stuff where performance doesn't matter. SSDs excel at everything else.