Last night I upgraded the operating system on my IBM Thinkpad R50e lap-top from Unbuntu 5.10 to Ubuntu 6.06 LTS (Long Term Support).
The whole process took less than an hour and was completely automatic except for one point where it asked me if I wanted it to delete some packages that were no longer maintained (I said yes as I didn't notice anything in the list that I used). There was only one minor problem (well, more of an annoyance) in that an icon I had set up for a shortcut to Idle (the Python IDE) must have got deleted which resulted in lots of "Can't find icon" error boxes. However, this was simply fixed by deleting the shortcut (I will set up a new one when I use Idle again).
The only differences that I have noticed after the upgrade are that the login screen is more glossy and Firefox is now at version 18.104.22.168 instead of 1.4.2. On the whole, I am very impressed with Ubuntu.