I've been using the current builds for ages now, here's my top reasons for using IE7 (beyond pure security and engineering):

  • Printing is better (there was this really nasty habit of truncating on the right margin)
  • Support for RSS and OPML in the browser.
  • SSL2 is off by default (yay!)
  • Really cool UI for warning users about certificate issues

http://www.microsoft.com/windows/ie/ie7/ie7betaredirect.mspx