I have OCZ Agility III SSD, it does make a difference as people have mentioned with regards to stuttering, but when you zone and that loading black screen comes up, its also fetching server data, so FFXIV isn't going to make full use of a SSD. Battlefield 3 for example can take a minute on a standard hard drive to load a map, on a SSD it takes around 5 seconds. Some games the optimisation of a SSD drive will notice considerably more than on XIV. But i would never personally go back to a normal HDD.
Seagate do a line of solid state hybrid drives, which are part SSD part HDD, and adapt to which apps you use the most so they get stored on the Solid State part of the drive etc.