A SSD is much faster when used as a boot drive. For example my 4 year old computer with win 7 used to take 45-55 seconds to boot. With a ssd it now takes about 10-20 seconds to boot.
I play a lot of games like ME3 and MW3. When the match started I would often be last in sometimes like 30 seconds late, becuase the game has to load the level and such from the disk. Now with windows and the game installed on the SSD, the map loads in like 5 seconds and Iam normally first in.
SSDs are fast for loading and copying files, but they will not make the game play or FPS faster.
The down side is they are still pretty small, so you really need a mechanical hard drive for all your personal stuff and you need to plan ahead in reguards to what programs you want to install on the ssd.
SSDs work best on newer boards that support AHCI, so if your board does not have AHCI then it might not even be worth the trouble.