If your browsers work OK, you do not need to disable IPv6. Also, there is nothing wrong with either Ubuntu (Karmic) or IPv6, so if you are experiencing any issues, they are because of your ISP / router / modem.
Everybody posts about applications to install after upgrading to Ubuntu 9.10 Karmic Koala. That's interesting, and some applications in those posts are quite useful. But those are just lists of applications like any other "Top 10 Ubuntu applications" list.
I've upgraded ever since Gutsy: to Hardy, Intrepid and Jaunty. But this time, I did a fresh install of Karmic Koala, given the new Grub, ext4 and so on. Here are some things (bugs I mean - with potential fixes) I've experienced running Ubuntu Karmic Koala as my main OS (for about 2 days now):
Ubuntu Karmic Koala 9.10 Beta was released today and includes the previously mentioned:
Ubuntu team is already planning for 9.10, which will see the light of day in October 2009. The desktop side will focus on beautification and an improved boot-up experience; the server side will target cloud computing.