My experience : Yes, I did get messed up pages initially after upgrade but a refresh sorted this out. I did have problems with Firefox (not IE7) and the Dashboard admin page where it would not display the wordpress notices stuff but complain about “The number of request header fields exceeds this server’s limit”. In “about:config”, I changed “network.http.sendRefererHeader” from 2 to 1 and this problem disappeared.
When writing, the visual option was not available. I am sure I was using this before but had to go to “Options,My Profile” to enable it. Then everything was fine. I probably fiddled for a couple of hours to achieve these 2 successes, fiddles I had never had to do before on previous upgrades.
I accept that by using free software, from time to time I may have to sweat over something but historically, the user community usually sorts everything out in the end, and no slower than many commercial vendors.
I live in a remote location so upgrading my server on the other side of the planet is a bit harder than it used to be. Note that I upgraded from 2.0.4 to 2.1
