What is most valuable?
Although Ubuntu is based on Debian Linux, it ships with (more or less) current software. In addition, there a are a lot of community-based repositories which can easily be added.
Since Ubuntu provides LTS releases with 5 years of support, you can have longer server lifecycles without having to worry about security patches.
One of the greatest things about Ubuntu is the possibility of upgrading to the newer release on the fly - no reinstallation is needed.
Other great things/features include: Great AppArmor support, huge official package repository from Canonical and the community, decent hardware support.
How has it helped my organization?
Ubuntu powers many servers in most of the companies I worked for. Maintaining them was - in most cases - easy and painless.
What needs improvement?
The rpm-based distributions (such as Suse, Red Hat, CentOS, Fedora) ship with support for awesome system management software (Spacewalk, just to name one example). Ubuntu often provides *some* support for these tools, but it is very rarely supported a 100%. Canonical has Landscape and some other stuff; however, this is not sufficient. Canonical clearly should provide more system management tools which integrate into enterprise environments. Red Hat, CentOS and Suse clearly have more advantages here.
Furthermore, the Unity desktop and the advertisment integration of third parties are horrible.
For how long have I used the solution?
I have been working with Ubuntu since 2010.
What was my experience with deployment of the solution?
Deployment was always straight-forward; however, upgrading to latest releases (e.g. from 12.04 to 14.04) could mean that you have to adapt your system management tools since some package names can change.
What do I think about the stability of the solution?
Not with Ubuntu itself, but some software being packaged by the community for Ubuntu is not built with optimum parameters. E.g. I had many struggles with the libvirt/KVM builds for Ubuntu.
Which solution did I use previously and why did I switch?
Before using Ubuntu, I was used to Debian and their concept of having rock-solid (but also very old) software in their repositories. Ubuntu, however, is not so extreme in this point: It is stable, but the provided software is not thaaaat old. In addition, Ubuntu/Canonical are eager to provide at least some management tools.
How was the initial setup?
The initial setup of Ubuntu is very easy.
Which other solutions did I evaluate?
I also like Red Hat, CentOS and Fedora; however, upgrading to latest releases without reinstalling the complete OS seems only to be painless with Ubuntu (please correct me if I am wrong, maybe it always went well for you?).
What other advice do I have?
Try Ubuntu. Seriously. If you are looking for a good Linux distribution with a large community and lots of packages, Ubuntu is the way to go. However, if you need management tools (e.g. for automated installation, patch management etc.) you might have to invest some time in getting open-source projects involved or by writing your own solutions (if you don't want to use Landscape).