Is Firefox 4 worth trying?

I use Firefox 3, have been using Firefox in general for years I saw something about a 4 once but am kind of nervous to try it. Does it work well with Windows 7? Some programs seem to be a bit temperamental with it, I know AIM is. Only one version that I have found works with Windows 7. I accidentally updated once forgetting this and was unable to open an IM window and had to downgrade again.

I don’t want to make this mistake with Firefox 4.
[Q]
So what is the verdict from other testers?