You wont get any in game effects, all of the data is offloaded to the RAM when it loads, you will however get a significant difference in boot and load times compared to a good 7200RPM drive.
I rarely turn my PC off, but I don't mind long boot times.
Load times are also fine, as I mostly play MMOS where the world is streamed.
This is the one thing that worries me the most though.
How would games with streaming worlds run? Something like GTA4 or Champions Online?