The best way to describe FareWatcher is to compare it to Southwest airlines Ding! program which sits in your Windows system tray (bottom right) and alerts you when an airfare becomes available at a price you specify.
I really didn't need another program running in my system tray, which is why I really like this extension. I'm heading down to Orlando, Florida in February next year and I'm looking for some good prices on some flights.
However, one problem that came up was that the extension will only work with Firefox 1.0-1.5. It would not install with the latest Firefox version (220.127.116.11). If the extension is updated to Firefox 2.0, then FareWatcher can provide the best airfare prices to you at a moments notice from multiple airlines as opposed to just one ("Ding!").
One feature that make the extension stand out is the notification method. Its similar to ForecastFox Enhanced. It scrolls up when it wants to tell you something and slowly scrolls back down. Click on it and you get the latest airfares for what you were looking for. This feature is only available in Windows, though.
Overall, I think this is a great idea and extension, but it needs updated for complete compatibility and possibly for Firefox 2.0.