It's good, it's stable and it's much faster than Firefox 3
However you might find that some of your extensions aren't compatible with it. (The installation program will tell you if you've got something that won't work under Firefox 4. For example, I had to get a different add-on for saving Youtube videos and also find a different way of capturing parts of a screenshot).
One or two things take a bit of getting used to. For example, if you right-click on a link, 'Open in new tab' is now the first option, rather than the second one. (For a while I kept opening a new window, rather than a new tab, because I was automatically clicking on the second option).
The 'Refresh' button has also moved. The 'Go' button, to the right of the URL is initially green. When you click on it (or hit 'Enter') it goes red and turns into the 'Stop' button until the page has loaded. Then it goes white and becomes the 'Refresh' button.
Chris