Stevietheman - 7:15 pm on Mar 17, 2011 (gmt 0)
IE9's standards compliance is definitely much better than before. Rounded corners and standard opacity, hurray! The downside is that you have to add an "if lte IE 8" conditional to use the old IE filters on IE8 and earlier.
(There's also a serious rendering bug introduced in IE8 that wasn't fixed in IE9RC, but I haven't tested with IE9RTM yet. I'll test again and report back as to specifics.)
If you're over 40, you likely won't like the miniaturized address bar buttons. One special issue is that "Compatibility View" highlights on hover with the same color as that for "Refresh", which encourages misclicks. Sure, I could click F5 for refresh, but on my laptop (like many others), clicking F5 means clicking a function key in tandem to get access. Clicking a browser button with my mouse is easier. And I want that button to be bigger and easier to distinguish from other buttons.
As far as performance, IE9 seems to load slower than IE8, but once it's loaded, it's faster than before.