They're supposed to be faster than most hard discs, but more importantly, any multi-threaded operating system can read/write multiple discs concurrently. So instead of the system needing to trade off with applications for the disc, this should take some pressure off the main drive, and, in theory, speed things up.

My laptop has a secure digital card reader, but my camera takes compact flash. I can get a fast 4 or 8 GB SD card pretty cheaply. I know they have a limited, but very large, number of reads and writes before they die, but if all I'm using it for is O/S and Photoshop scratch space, loosing the data would hardly be catastrophic.

Has anyone tried it? Am I missing anything? And is it worth the trouble in reality?

Not tried it with Photoshop specifically, but we used CF cards as data storage for an embedded OS application with no known problems over a couple of years.
I'm assuming it's still operational as we have had no contact with the client in about 12 months.