There are a lot of changes under the hod. The release candidate adds support for CSS3 2D Transforms, HTML5 Geolocation, a set of HTML5 semantic elements, and the HTML5 canvas globalCompositeOperation property. They also improved the performance of CanvasPixelArray.
"The IE9 RC is faster with real world sites. In addition to making the script engine faster, we've improved and tuned the rest of the browser as well. You'll find that Gmail, Office Web Applications, and many other sites are faster as a result of scenario tuning, network cache tuning, and new compiler optimizations," details Dean Hachamovitch, Microsoft's corporate vice president for Internet Explorer, "You'll also find that the RC of IE9 often uses megabytes less memory than the beta because of changes like delayed image decoding. We've also improved the performance of things many people do every day, like find on page, and made improvements which extend battery life."
Interface-wise, the biggest change is the addition of an option to display the tab bar on an additional row. Sadly, you can't place these tabs atop the address bar where it belongs according to the Gospel Of Chrome (the One True browser interface). Another issue is the font rendering, which is still fuzzy due to the DirectWrite support. Why IE9 can't just use my global font settings is beyond me. It might sound petty to some, but it's frickin' 2011 and I demand consistent font rendering. Whether it's the fuzzy but shape-accurate Mac OS X variant, or the sharp but shape-inaccurate variant in Windows - they're both fine as long as it's an either-or thing!
You can download the release candidate today.