I never used it, but one point against using it is certainly that it's currently at alpha stage, there's basically no documentation and Android support is broken.
From the presentation video it seems like audio support/capabilities is kinda bad. Also there's no socket support (yet).
As with every language that provides a high abstraction layer, you're going to sacrifice some performance. In this special case a lot depends on the "compiler" that will generate the native code for the platform. No problem for Android, since Java is already the native language there, the JavaScript compiler seems to be very good, just the Flash output is not at a level that is comparable to a native app.
You'll also have to consider that to create a game that actually runs on all platforms by simply swapping the "main" class (as demonstrated in the video), you're also going to have to restrict yourself to the common capabilities of the target platforms. That might mean that you cannot use multi-touch as a core gameplay element, or keyboard input etc. unless you start to code some platform specific bits. But that's a problem all cross-platform developers are going to face.
These are some "generic" cons to consider. But the project looks interesting and could provide a great platform to deploy to HTML5, Android, Desktop and Flash once it enters a more mature stage.
