I actually haven't used the Firefox 3 Beta much yet, beyond the initial "download it, look at it, drag it to the trash" thing that I always do. But in my experience, Mozilla apps are very stable by the time they get to the later beta stages--I used the Firefox 2 Beta for months before it was officially released.
On the other hand, a lot of add-on authors don't update their extensions to the latest version until the final release. They apparently miss the fact that, as developers, it's them who are supposed to be testing their add-ons against the beta versions. But, I digress. Anyway, I don't use many addons (mainly the User Agent Switcher and ForecastFox), so this isn't too big of a trade-off for me.
In other words, if you want it, go for it.