I haven't bothered installing it yet myself. I was put off by the beta I tested a couple of months ago. The layout changes did not appeal to me at all. But what I found interesting in the BBC article was this little nugget:
Firefox 4 was made available for download less than a month after Microsoft launched Internet Explorer 9, the latest version of its market-leading browser.
Ha ha, that was a stealth IE release, I never heard anything about it, nor met anyone who gave a rat's arse.