I disagree with the accepted answer in practice.
It's clear that if you put some files on slower storage than SSD, you will clearly have slower access times for those files, but for speed in real usage for OS X 10.7 and later - the vast majority of the speed gains come from SSD backing the OS and the applications and the system libraries and caches.
The nature of large file serial IO is that when you are reading or writing to movies, videos, photo image files and other items - HDD actually perform quite well and the system knows how to optimize and wait for serial IO to finish without blocking the user. Similarly, read-ahead buffers are great for managing known/predictable access patterns.
SSD excel in other areas and as long as you don't have a system that's under enormous RAM memory pressure - you can run with all user files on the HDD and the rest of the files on SSD quite happily and realize great speedups over having all files on HDD.
Some exceptions to the general rule would be Virtual Machine storage - it might be worth copying them from HDD to SSD if you have room to run a VM out of the SSD.
I have some systems where there are almost a TB of VM images total, but I only need 100 GB of VM running at any one time and will move the images to SSD before starting the hypervisor software when the performance benefit for that use merits me caring about where specific files are stored.