I recently upgraded my Ubuntu 9.10 installation to Ubuntu 10.04 LTS. At the outset of the process I was really hoping that I could just start the upgrade, leave my computer running over night, and come back to find that the upgrade all finished. Unfortunately, I was wrong. Throughout the upgrade process there were several points where I needed to decide whether I wanted to keep some old configuration file or overwrite it with something completely new. Seeing as how I kept leaving my computer as it was upgrading (in the hope that it would finish automatically) only to come back and find that it wanted me to tell it what to do at these decision points, the whole process took about a day!
While it was certainly annoying to get through the upgrade, I'm very happy to report that absolutely nothing cataclysmic or annoying happened to the programs I had installed in my 9.10 installation or the data that I had accumulated. Everything's running very smoothly: My OpenSSH server, Alpine, PostgreSQL, R, Python, Firefox and Thunderbird, and so forth.
The one thing that kind of annoyed me was the messaging menu. It's a little envelope icon that sits next to the volume icon in the Indicator Applet. I'm not really that into microblogging, so I don't have a use for a panel applet that facilitates such functionality. Thankfully, I learned a way of getting rid of it. Just go into your terminal and type in sudo apt-get remove indicator-messages. Once the process completes, you right click on the indicator applet (the volume icon and the envelope icon together), click on "Remove From Panel", then right click where the two icons previously were, click on "Add to Panel...", highlight the Indicator Applet and then click on "Add". Voila! The messaging menu is gone :)
I've read numerous times of people having problems upgrading Ubuntu, versus doing a fresh install of a new version. Thankfully in the case of upgrading to Lucid Lynx from Karmic Koala, I have seen no issues! So, I heartily reccomend upgrading to the latest and greatest version of Ubuntu.
I mentioned previously that Ubuntu 9.10 tended to have problems trying to suspend my Toshiba Satellite A200 laptop. I'm happy to report that Ubuntu 10.04 does not have such problems.