I disagree with other peoples posts that state that gamers don't benefit as much from an SSD. I have a 120GB Vertex 2E which is more than enough to install Windows along with usual apps and some development programs with room left over for a number of games. A lot of people on here have high performance systems and already have high end graphics cards/cpus. The addition of an SSD basically eliminates load times for loading a game and in game loading between levels. This is great!
On top of this, I play Bad Company 2 a lot online and if the level changes and the server has the spawn time set to 0seconds, I am nearlly always the first person who spawns into the game (thanks to the quicker loading time) giving me a slight advantage!
Overall SSDs are so much quicker at loading applications, installing programs (from installers on the SSD) and anythign that requires reading/writing of data such as extracting files. The biggest bottle neck in a computer is likely to be the mechanical drive. This is the one upgrade that will actually make a difference.
It's still a bit of a luxury, you're using up 5-10GB of space per game to shave a few seconds from loading times, which is a pretty poor return at current prices. I'd class SSD's as essential for a boot drive+apps in any new build. For games though, just run a Raid0 of Samsung F4's or similar fast mechanical drives. Hypocritical since I've splurged on 160GB worth, but I'm lazy - could easily get by on half that with a little management.