I've been using the beta and RC versions for some time, Lona, so I've almost forgotten what 1.5 was like! It's not much different on the surface, I don't think, but obviously there have been some improvements under the bonnet, and there are some changes in the Options. What you may find is that some of the 1.5 add-ons might not work, until the various developers update them, but I have found most of them have now caught up, and I think there's only the odd Theme which isn't compatible. I have to say, it isn't my default browser, as I still use the Mozilla (now SeaMonkey) Suite, but I would certainly recommend updating from 1.5.