Thanks for the feedback.
So the consensus so far is:
SSD: OS and Apps
HD: Home directory and general storage
Any other suggestions?
If at all possible, split the "big file" parts of the home directory (iTunes library, videos, etc) from the core parts (the 'Library' directory, for instance). You want any files that are going to be accessed frequently and quasi-randomly to be on the SSD (as the performance gain is xbawks-hueg) rather than on the HD.
I can't speak to the performance of the BTO SSD, but in my 2010 MBP I get about 15MB/s from an OWC SSD for random 4k reads (the sort of stuff cache does, or loading prefs on startup). Sounds crappy, until you realize that the 5400rpm drive the SSD replaced could *barely* manage 1MB/s under that same load - the heads spent most of their time moving around, not transferring data.
Probably not relevant for the iMac, but one other advantage this offers is that the HD can spin down when not in use (when iTunes isn't playing anything, etc) - there's a noticeable pause when it's first accessed, but it does save some battery life to not keep it spinning.
Overall, here's a simple guide
- on the SSD: files that are accessed randomly and/or that will block the system waiting for them to be read
- on the HD: files that are mostly read sequentially, bigger files. Conveniently, those two categories are nearly identical for most workloads (doing heavy NLE on uncompressed HD video is a typical exception - big files, accessed somewhat randomly).
