I've been a Firefox fan for years -- I load it on every machine that I sue, regardless of operating system (and usually regardless of whose machine it is). Generally I don't get into playing with the beta versions of Firefox, but due to the amount of buzz around Firefox 3 beta 4, I went ahead and installed it.
First things first, and this may be the reason you don't want to install the beta: most of the add-ons that I use were disabled as they are incompatible. Luckily Greasemonkey is compatible and most of the Greasemonkey scripts are compatible, but things like Google Toolbar and the del.icio.us toolbar haven't been updated; while I don't mind some of the things not working, the del.icio.us toolbar is what I miss the most. (P.S. -- I did install the nightly tester tools to override and enable all the extension, which was a totally bad idea as Firefox got into an infinite loop where it wouldn't boot -- luckily I found out how to launch it in safe mode from the Terminal in OS X and disable all the add-ons).
Here's what's cool about Firefox 3: it is really fast. You can search around for benchmarking and read all the reviews, but the simple fact of the matter is that I can observe it rendering and loading pages faster. Further, it seems to interact in a more snappy manner with my heavy-use Google services such as Gmail and Reader. I also like the history that comes up as I'm typing in a site.
Interestingly the search box did not install by default and I never realized how much I used the search from the toolbar until it wasn't there. Luckily that was easily fixed by simply dragging and dropping it on the toolbar. I'm guessing that if the Google toolbar was installed, that would have been automagically taken care of.
The Firefox 3 interface is cleaner and tighter and the buttons more closely resemble the OSX interface, which is quite nice.
- If you are a user dependent on your add-ons, wait for the actual release, which is supposed to be June some time.
- If you are nervous about being bleeding edge and beta scares you, then wait for the full release.
- If you are like me and get along without most of your add-ons and love relatively stable bleeding edge stuff, then download it by all means.