It's a release candidate today, not the final product.
IE8 has a couple of nice features: accelerators (the ability to highlight a word or phrase and perform some action on it, such as translation), and colored tabs: spawned tabs are the same color as the parent window. Webslices are also interesting: you can, say, subscribe to part of a website and track changes. The address bar is still a little behind FF3. Zooming seems to retain the page layout slightly better in IE8. Quick Tabs works very well, especially with the ability to save/reload tab groups, etc.
In comparison to FF3? Maybe a slightly better experience out of the box. I've got IE8 on some machines, FF3 on others. I don't really care one way or the other.