I just upgraded to Firefox 2 and was impressed with how seamless it was. Double click the installer, follow the prompt to update a few extensions, and I'm done. My browser looks the same, it still has all my history and stored passwords and remembered forms, it works the same. Little fiddling required. And with Google Browser Sync I'm even able to move between my laptop and desktop and have things follow me around.
Contrast that to FeedDemon, which after an upgrade requires me to go through again and configure it to not act dumb. Or World of Warcraft; today is addon upgrade hell as the new patch comes out. And managing client state between machines? Total pain.
One problem with Firefox 2.0: what's up with the gradient fill on the tab bar? Hideous to have that embedded in my otherwise flat-coloured chrome.