According to Trustworthy Computer Director for Microsoft Yunsun Wee, the Fix It tool "is an easy, one-click solution that will help protect your computer right away. It will not affect your ability to browse the web, and it does not require a reboot of your computer. This will not only reinforce the issue that the Fix It addressed, but cover other issues as well, including four other critical remote code execution issues."
This isn't some minor hole, however. In fact, the vulnerability is so severe that the German government and security experts have been advising people to avoid using Internet Explorer altogether until the vulnerability is patched entirely, which won't happen until the larger update is released. That update, which has a severity rating of Critical, should come out any day now, meaning users won't have long to wait until they can get back to viewing the internet in peace.
This vulnerability, more specifically, was uncovered within the past week and could very well compromise the PCs of every single Internet Explorer user who visits a malicious site. The flaw itself is being actively exploited to deliver a back-door Trojan known as Poison Ivy. My recommendation is that you download the patch as soon as it is available so as to not risk any critical harm befalling your PC.
Source: CNET - Microsoft issues fix for IE hole; full update coming Friday