I'm watching the SSD marketplace very closely, and everything I read above still convinces me to stay with HDDs, until the SSD world improves a LOT!
Now hear this ...
Consider a RAID Edition 5 ("RE5") line of WDC HDDs.
Here's why: I can't find a 6G WD HDD with 64MB cache that also supports their Time-Limited Error Recovery ("TLER") which they highly recommend for all RAID arrays, unless I buy their expensive SAS HDDs, or spend a lot on Velociraptors.
For example, at the WD website they warn against using multiple Caviar Blacks in a RAID array, because those HDDs don't support TLER. The other reason is that, generally, higher performance comes with higher cost and with higher capacity too.
So, imagine a WD RE5 HDD with 64MB or 128MB cache, 6G interface, 7,200 rpm, and perpendicular magnetic recording ("PMR"), but in relatively small capacities e.g. 64GB, 128GB and 250GB -- targeting the SOHO market sector (initially).
[ "SOHO" = Small Office / Home Office ]
With the right pricing, 4 x HDD caches @ 64MB = 256MB in RAID 0 (HDD caches are effectively additive in RAID 0 mode).
The closest I can come to this goal is the WD1500HLHX:
I would jump tomorrow for an inexpensive 64 or 128 GB version with PMR, 7,200 rpm, and either a 64MB or 128MB cache, as long as its interface were also the 6G standard (600 MB/second).
This "RE5" series would be designed specifically for RAID arrays where TLER is recommended, and where the combined HDD cache total creates a large effective hardware buffer that helps to eliminate the need for a dedicated I/O buffer on-board expensive RAID controllers.
Now, let's look at some realistic prices: the WD2503ABYX retails for only $70 at Newegg:
Let's reduce the capacity to 64GB or 128GB, keep the cache at 64MB, keep TLER, and up the interface to 6G: WD should be able to sell such a "RE5" HDD at an MSRP at or near $75 USD.
So, 2 of these in RAID 0 cost ~$150 and should exceed 290 MB/second sustained. 4 of these in RAID 0 cost ~$300 and should exceed 500 MB/second sustained.
Our experimentation here was able to reach 618.5 MB/second, but that speed required 4 x Hitachi 15,000 rpm 2.5" SAS HDDs at $250 each ($1,000 just for HDDs and not counting the additional cost of a Highpoint 2720 controller):
Summary: Until SSD technology matures a lot more, RAID arrays of relatively small but relatively fast rotating HDDs can perform quite admirably for much less money and with much greater reliability than current SSDs are able to demonstrate. This kind of product would be very appealing to the SOHO marketplace who are understandably wary about paying high prices for relatively poor reliability.
P.S. Just one man's opinion here: your responses are all welcome!