For all its merits and potential downfalls, one area IE 7.0 is not having trouble in is getting onto the desktop. Now, the new browser from MS has over 100 Million installs. The article doesn't mention whether or not it factors in those who were participating in the Vista beta, though likely that would be a minority either way. Interestingly enough, it seems that the installs of IE7 are primarily coming at the cost of IE6. Worldwide usage of Firefox seems to be remaining stable, and IE6 is dipping to all-time lows:

IE7 and Firefox still don't quite have the muster to take down the perennial browser leader: Internet Explorer 6. IE6, which was first released on August 27, 2001 is still the leading browser on the market with around 62% marketshare.
Given the benefits IE7 brings over IE6 and its inclusion in Vista, and the nature of IE's history, upgrading is the best course of action for IE6 users regardless. Even so, it's happening fast.