I've been using the IE9 beta since it was released and find it to be mostly excellent bar some (understandable given it isn't final) bugs. The 64-bit build was especially prone to crashing with the 64-bit Flash Player "square" preview but the 32-bit version has proven to be very stable.
It is certainly fast on my machine, slower than Chrome but faster than Firefox 3.6.13 and 4.0 beta 11. I don't like Chrome much though hence it isn't my main browser. The way Chrome displays and downloads stuff is clumsy (it's much better in Firefox and especially IE9 as that has a Save As option) plus the location of the bookmarks on the left irritates me (yes, you can use the bookmark bar but it can only hold so many). IE9 strikes a nice balance between Firefox and Chrome whilst offering a nicer interface IMO. That's why it is now my preferred browser of choice.
My only niggles with IE9, and earlier versions of IE too, is that it doesn't seem to like remembering usernames and passwords for the regular sites I visit unlike Firefox or Chrome. It seems to remember them for a while (after prompting me to save the details) then a few days later forgets them for no logical reason. Firefox has an excellent Password Manager in case I forget my username or password but, ironically, doesn't really need it as it always retains passwords/usernames for sites I've opted to remember my details for. IE could really do with such a feature as well. Sure, it is a security risk but I'm the only user of my PC anyway and Firefox locks its Password Manager with a password anyway.
Also IE9 like IE8 doesn't always display websites properly even with the compatibility mode enabled. I've always found this strange considering that IE is the most wildly used browser available. True, it applies to only a handful of sites but Firefox and Chrome don't seem to have any issues at all.
