I am using it for some days and it is really cool as it has 3 different interfaces for Windows, Mac OS X and Linux respectively. Also I am amazed by the small installer size of Firefox 3 beta 3 and Firefox 2.0.0.12, as they are notably small from their previous counterparts.
Performance is good and eventually better than the previous builds and stability is also better. The higher RAM usage is a bit improved from the Firefox 2 but with few tabs open together it is still using a lot of memory. But nonetheless I am happy with the performance.