This release comes with a revamped preference panel, tabbed browsing enhancements, loads of bugfixes, faster back- and forward, … in short it just got better. How much, you’ll have to find out yourself.
BUT If you plan to upgrade, please make sure none of your critical extensions break. If they do, persuade the original extension creator to fix his/her extension ASAP. Or take a look at the Mozilla Quality post: “Some help testing Extensions for Firefox 1.5“.
Personally, the only extension that hasn’t been updated for me, are the Furl Tools.
On a sidenote: I expect the Portable Firefox version to be released very soon.
Update: This final release seems to have the same timestamp of FF RC3. Which some of us have been running since November 11th. I must admit, I have not run into issues since. Some extensions reserved.
And it seems mozilla.com is favoured over mozilla.org. I see former links pointing to .org redirected to the .com.