Seconded. I have the X-25 my MBP. It is probably faster than what apple is putting in the airs, but I haven't checked. 2.33 GHz c2d, 3 GB ram. I still have a lot of trouble doing things like heavy multitasking, full screen web video or viewing pages heavy in JS/Flash.
SSD isn't a cure all. It's good for booting up, loading large applications, and when you're low on ram.
It really improves boot and shutdown. This Mac starts up almost instantly. Shutting down takes only a few seconds.
It improves compilation of native code a bit, but not a lot. Macport installs are slightly faster. It doesn't really improve Java compiler results.
It also helps when you're running your machine at its memory capacity. I never hit swap hell anymore.
It does improve things, but I wouldn't trade the 2.6GHz CPU in this for the much-less-powerful 1.8GHz CPU that's top of the line for the Air.