Absolutley yes. In fact Ubuntu 8.04 LTS is still supported until April at least and even then the Server Edition will be supported until April 2013 therefore any critical updates should make it through to the Desktop Edition. I have a custom LiveDVD of Linux Mint 5 based on Ubuntu 8.04 LTS that I run occasionally and its a fine release.
Ubuntu 8.04 LTS is mega stable with the release revisions (now 8.04.4) In fact Ubuntu 10.04 LTS is only just catching up stabilty wise (now 10.04.2)
As long as the repositories are active its a case of 'if it ain't broke don't fix it' Its not until you start having to use the old-release repositories it is time to move on.
Current supported releases are
Ubuntu 8.04 LTS
Ubuntu 9.10 (support ends in April)
Ubuntu 10.04 LTS
Ubuntu 10.10
Ubuntu 11.04 (in development)
Ubuntu 9.04 is no longer supported but its repositories are still active. Not quite sure how long for though.
Ubuntu 8.10 is now using old-release repositories
Anything earlier than Ubuntu 8.04 LTS and things start getting a little more tricky as you need to completely replace the repositories with old-release ones
For example to work with Ubuntu 7.10 (Gutsy) you would need to do the following
Open a Terminal and submit the following command
sudo gedit /etc/apt/sources.list
The sources Window will then open. Delete all the Ubuntu 7.10 'gutsy' repositories and replace them with these (copy and paste)
deb http://old-releases.ubuntu.com/ubuntu/ gutsy main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu/ gutsy-updates main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu/ gutsy-security main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu/ gutsy-backports main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu/ gutsy-backports main/debian-installer
deb-src http://old-releases.ubuntu.com/ubuntu/ gutsy-backports main restricted universe multiverse
Once you have done this go to the top left hand corner and click save
Go back in to the Terminal and do
sudo apt-get update
It is most likely a considerable number of updates will then need to be applied (you will be notified by the Update Manager)
Ubuntu 7.10 (Gutsy Gibbon) User Guide
http://ubuntuguide.org/wiki/Ubuntu:Gutsy
I had to do the similar with Linux Mint 3.0 and Linux Mint 3.1 though instead of (Ubuntu 7.10) the old-release repositories were for (Ubuntu 7.04)
Although Ubuntu 6.10 was decent enough anything earlier than Ubuntu 7.04 just isn't worth the hassle
LUg.