I did it on my 2008 24″ iMac. Used the SSD as a boot drive and for stuff I wanted fast disk access to (applications and some videos mainly). Kept the mechanical disk in the chassis and used that to store downloads, documents and ‘stuff’
You can set it all up with symbolic links in such a way that OS X doesn’t really feel any different to single disk setup.
In terms of performance it was fine for my needs.
I eventually sold that iMac without the SSD for £500 and bought a second hand 27″2012 iMac i7 which now has the SSD installed. The 27″ cost me an extra £600 on top of what I sold the 24″ for and to be honest it wasn’t really worth the upgrade – there was nothing wrong with the 2008 24″ c2 duo
The hardware / screwdrivery bit is fine if you take a bit of time and follow the youtube or ifixit guides.
I see apple have something called a ‘fusion drive’ now supported from OSX 10.8.2. It’s basically an HDD joined with an SSD at HW level. OS X sees only a single disk as opposed to 2x disks with the SSD_HDD setup.
The advantage of this setup is that OS X sees it as a single drive and has clever code that will automatically move objects that could benefit from performance into the SSD part of the drive and archives stuff that is not used regularly to the HDD part of the drive. (In my current setup I have hard coded OS X, apps and some videos to the SDD with fusion drive OS X would sort it all out for me a fusion drive would be more flexible and possibly more performant)
There are some guides as to how to DIY one.
DIY fusion drive
hope this helps