I realise it is still in beta but you should have an idea about stability to give me a general idea at this stage. If it is not worth it I guess I'll stick to Firefox 2.0.0.12.
its really hard to answer your question since we all have different surfing habit, however, so far for me, firefox 3 beta/nightly is very solid.
I think you should try out for yourself after beta 4 rolls out.