One mac, Two or more storage devices - how to divide things? We're at a crossroads where most Macs now have space and connections to contain two storage devices. SSD are fast, low capacity and expensive - HDD are in comparison slow, huge and cheap. 

Both iMacs and Mac Mini can ship configured with two drives and all the portable Macs that still ship with an optical drive have a choice of several professional kits that securely mount either a second HDD or SSD inside the unibody. (I've even seen Airs with a light USB drive semi-permanently affixed to the metal side of the display with velcro since sometimes even the slimmest Mac needs to pack more storage punch.)

Yet Mac OS X doesn't really have a Setup Assistant that's willing to help divvy up our data on two drives. Software to intelligently move files to appropriate storage exists in the enterprise SAN space, but I don't think we'll see it for a while on the desktop. There's not even a guide I've seen on what or how to set up a mac with two drives, so Apple have left us a clean playing field upon which to experiment.

The best writing on this topic I have seen recently was written by Matt Legend Gemmell. His article titled Using OS X with an SSD plus HDD setup is great reading and seems to be relevant even if you have two spinning HDD.

Matt advocates 


installing the OS, user home folders and most user data on the SSD 
place user data that won't fit on the SSD instead on the HDD
implementing this by moving entire top level folders from the SSD home folder and placing them on the HDD. 
using soft (sym) links to connect the two locations.


Is this thinking the current state of the art? 

What works for you and more importantly, why does it work for you? 

Tips on how full or empty to keep your boot drive and any gotchas you learned the hard way are greatly appreciated.
My setup is the following : 


Macbook pro 2009, 13"
SSD (Intel X25M Postville) replacing the superdrive, used as system + home folder disk
Factory HDD used for music and pictures storage


I tried to move my home folder to te HDD, here are my conclusions : 
when you move your entire home folder to an HDD, your mac boots fast, but you don't get too much performance gain while opening applications (even apps that do not use any document, like safari). That's because when the app starts up, it loads it preference and cache files from your library. If it is on the second drive, startup of apps is nearly as slow as when  you use your HDD for booting. 

I now use my HDD for iTunes/Aperture and vmware images only. These apps are a bit slower than the others (but still as fast are when I didn't have a SSD). But I did not do as Matt recommends : I created new iTunes and aperture libraries directly on the HDD, using the alt key at launch. I do not like symlinks, that can break if you unmount your HDD, for maintenance or by mistake for example, leaving your home folder in a non consistent state.

Last points : 


with this configuration, you have to be careful of what files you put on the HDD if you have multiple accounts on your mac : these files are accessible to anybody by default, and filevault will not protect them (unless you use Lion which allows full-drive encryption). 
And you need to setup Time machine to include the HDD (if needed), which includes only the system drive by default.