Depends on the style of game really.
I keep most of my games on mechanical drives. For games where there are constant little loads from disc, its not that big a deal. Where I did see a difference was when playing MMOs - I would find my character would zones much quicker, especially when entering a raidzone.
But I wouldn't say an SSD changed my MMO playing experience that much. Yeah its good for the kudos of being first, but if you still have to wait for others to load up.. does it really make that much difference?
Don't get me wrong, I wouldnt willingly go back to running Windows and apps off a mechanical drive anymore unless I had to. Its just so snappy and responsive in comparison.
There are utilities to move programs from SSD to HD and back again as well if you dont want to manually install things to your D: drive (for example).