Windows has already pushed the IE 9 update, at least it has on my machines, through its auto update feature. From tests, it looks like the 32 bit version is ok, but 64 bit is slow. FF 4, and Chrome 10 are faster and more secure,still.
But like everything in computing, it is personal choice. Since it is only available on Windows 7,and Vista, XP users or Linux are left out, so if you are looking for a consistent browsing experience in a mixed environment, I would choose something else. Personally I like FF 4, and the share bookmarks automatically between my desktop and my laptop, is brilliant.