It could be an exciting phone, but from the demo video two or three times there was tearing/slugging happening while using it. Also one or two times it was not responsive to the user touch.
The demo you're seeing is running a version of Firefox OS based on Firefox 18, which is quite old now. Soon, version 1.1 will be released which will include some optimizations to the front end and help improve performance. By the end of the year, though, you can expect an OTA update to be released ("B2G 1.2") which is based on Firefox 26 (the current equivalent of Firefox Nightly). The performance improvements you'll see will be very noticeable.
The version of FXOS that's presently shipping has a lot of rough edges. The B2G 26 branch includes all of the changes made between Firefox 18 and Firefox 26, many of which are performance related. Notably, JS performance is going to shoot up (new Spidermonkey goodness), SVG performance is going to get a boost (better hardware acceleration, better caching), and lots of general optimizations are going to land.
I'd like to think of the current FXOS builds as the Android 1.6 of the product, and the forthcoming versions as the Android 2.2/2.3s of the product.
