I upgraded to WordPress 2.1 this morning and the whole process was remarkably painless.
The entire process took about ten minutes – the majority of that time was spent on backing up the previous version and FTP’ing the new files into place.
Of course, if you are a client of mine and I maintain your blogs, it actually took waaaaaaaaaaaay longer. Hours of research, planning, etc. were involved and if you need me to upgrade your blog, my invoices would obviously reflect that 😉
The only glitch I found so far was that when I was double-clicking a word to select it in the Visual mode, the paragraph I had just been writing disappeared. And that disappearance was then auto-saved!
Clicking into the Code tab revealed that I had somehow inadvertently switched the paragraph style to hidden. Deleting the style info revealed the text once more.

I like the auto-save feature (see in the image above, the post is being saved) and the ability to quickly move between the Code and Visual (wysiwyg) views.
The builtin spell checker will prove handy too when I’m not using Firefox (Firefox 2 also has a spell checker which is fabulous for illiterates like me) or the Visual editor.
The other very nifty improvement I noticed was the ability to set any page as the front page of your site. This will allow many more people to use WordPress as a simple CMS for their entire website.
WordPress just gets better and better, great job guys.