This morning I upgraded my laptop operating system from Ubuntu 8.10 to 9.04. All went smoothly and took under 2 hours (less than I feared). I don't know exactly how long it took because I went out shopping while it was under way (there's confidence for you!) and it was waiting at the "Remove unneeded packages?" screen when I got back.
I have only noticed one problem following the upgrade and that was the graphics in Blender no longer work. However, as I no longer use Blender on the laptop, I just uninstalled it. (Blender is listed as a packge from the "Universe" and so is not officially supported by Canonical Ltd.)