I would say yes, it seems to create better optimised files than photoshop. I have them both and i tend to use photoshop for design and fireworks for the optimisation, though the difference between the two for optimisation is marginal..
a little side question, for those of you who do use fireworks, have you noticed that it seems to handle large amounts of text badly? it seems to scoff my system resources and runs really slow when large amounts of text are involved.. (as compared to photoshop etc). - or perhaps my system is too pants.