I really love IE9. Where performance is concerned, it is to prior versions of IE what Windows 7 is to prior versions of windows.
However, it acts all glitchy on certain sites unless you go to compatibility mode. For example, on these StackEchange sites, I have to go into compatibility mode to mark something as an answer. On Facebook, when I click the comment link on someone's post, it doesn't set focus to the textarea element on the page. I am also a web developer and have been introduced to a host of browser issues with IE9 in regards to our third-party web development tools. These are just some examples.
My theory is that IE9 is actually doing things correctly now -- at least more correctly -- and all the IE9 hacks that developers have had to implement on their sites to get IE to work right are now breaking. It might seem like a stretch to think that Microsoft have finished doing things their own way, but perhaps they've finally got the message?
Anyway, what's up with IE9's odd behavior on some sites now? Anyone else notice these issues?