in my opinion, an SSD is totally worth it. i tried upgrading to an HDD with higher cache... the gain was nominal. i have tried an SSHD... again, the gain was very nominal. i put an SSD in the PS4 and FFXIV was loading super fast. warp times went from like 30~45seconds all the way down to like 10~12seconds. i would consider this a huge gain.

there are other areas of improvement besides load times. but they don't seem to as pronounced on the PS4 as they were on the PS3. probably due to better hardware.

i don't think this was mentioned, but you need to have at least a 240GB drive for the PS4 to utilize it. also, when shopping for an SSD... you don't need to get the best one on the shelf. the PS4 isn't able to utilize the full potential of an SSD, but it can utilize enough to make a significant difference.