I recently "took the plunge" and changed to FF3, without drama. Couple of slightly quirky things happened, and a couple of browser crashes, hasn't happened since updating again to 3.0.5.
All bookmarks/settings carried over just fine.
This last update of FF2 will be the last; it is no longer going to be supported. Basically means that any vulnerabilities that are discovered in the future in FF2, you'll have to learn about and mitigate for yourself (if possible.)