I've upgraded Ubuntu 9.04 to 9.10, and here is what I found about it.

At upgrade start, it asked about keyboard. Surprisingly, it has nothing about Brazilian keyboards and I choose to maintain "kernel keymaps" without know what exactly that is and why that keyboard popup appeared.

During the upgrade, flash_plugin and package_hook updates crashed. Nvidia driver, as always failed too. So after restarting in safe mode, I changed nvidia driver to nv to be able to enter in the graphical mode.

Then, to fix the system:
    sudo aptitude safe-upgrade
    sudo aptitude safe-upgrade
    sudo aptitude dist-upgrade

And reverted to nvidia driver.

They claim to have new beautiful and faster boot. Lies. The new boot screen is horrible, specially when running fsck. And appears to be slower than in 9.04.

The "fast user switch" gnome applet do not exist as before. It can't show your IM status anymore, nor can be compact as before. I changed it by the shutdown applet to have the same functionality that I used to have.

As with Ubuntu 9.04, Firefox 3.5 continues slow as the 9.04 Shiretoko in my system. And the flash plugin continue crashing. I use the AMD64 version, which requires wrapper to 32 bit flash, because Adobe didn't released the final AMD64 version yet.

Resuming, you decide. I want an updated system anyway. :-)