SSD is most noticeable on startup, when it makes a huge difference. Faster boot of a VM, too. Makes sense, at startup, the OS needs to read LOTS of files. I think it would also be incredibly helpful for something like video editing where you're constantly doing big read/writes. Of course for that you'd also run out of space fast
From everything I read a month or two, just be sure that the SSD you choose has the Sandforce controller.
If you want to get fancy with the secondary drive, get a simple 2 drive mirrored RAID.