There are pros and cons to upgrading to 6.0. My advice, if 5.5 is working for you, don't upgrade.
I personally continue with the 5.5 as I find no problems with my websites with that version.
With the 6.0 version at the office, there are several websites that do not want to load or do not want to display the pop-ups correctly. The 6.0 service pack that was released has helped considerably with the java issues, but there are still annoyanaces. Sometimes IE will stop responding, other times, it doesn't wish to print, and my favorite, it closes automatically.