It doesn't seem to notify you if you use "Check for updates..." in the older version so go ahead and grab it if you haven't already! It may have its own fair share of vulnerabilities but I'm sure there aren't as many hackers targeting it since IE remains popular as it comes pre-installed with PCs.
I've actually been using the RC3 version for a while now the test version and I've been enjoying the built-in spell checker along with the slightly changed interface. I prefer the old way of closing tabs though with only one 'X' at the right hand side. The spell checker's handy for catching those typos but by default it seems to be American English only. You'll have to download the British dictionary file.
What I still miss from IE is the searchable cache feature so that you can find a site you came across before easily. I've yet to find an add-on that lets you do that. The "Go" menu's been changed to "History" now but it still seems very slow when your History setting is greater than say 30 days. Once that's sorted, it'll be the perfect browser for me! :P
Check out the other new features in the release notes.
Opera remains the fastest (or smoothest) browser I've ever used however. Websites feel like they're just gliding across a frictionless screen (figuratively speaking, of course)...