It leaves every 30 minutes, and takes you straight through to Termini or the airport itself. Journey time is about 30 minutes.
Its one drawback, for small people like me, is the height of the carriage from the platform. Not a problem in itself, but a real nuisance if you have to cope with a heavy-ish suitcase as well. There's not a massive amount of luggage space inside either, although reasonably-sized suitcases will go under the seats.
There's aticket booth at the airport station, in front of the platform. At Termini there is also a little booth on the Leonardo Express platform (24, well-signed) but if the man isn't there you can easily get your tickets from the machine (in other photo).
I think the convenience of this train probably outweighs the fact that it is double the fare of the 'normal' stopping train.
Don't forget to validate your ticket using the yellow machines on the platforms before you get on the train.

