I pulled down the tarball a couple of days ago, installed it, and had only a few problems, all related to extension bugs (e.g., "adblock" needed to be installed from the adblock home site for a bug fix not yet in the Mozilla repository, and my favorite theme ("Silver Skin") is not yet converted to 1.5).
But I've had no
problems from FC4 while using FF 1.5.
Edit: Opps! Just found one: I just tried to open a new FF (using a different profile, in a seperate session), and FF wouldn't let me do it. I suspect
that this is a "feature" in 1.5. But it's somewhat anoying.