Recs.
Updated
SpecsUpdate
Pros
Pro Great community support
As the most popular Linux distribution, there's a wide range of sources for support online if you ever need help, including the Ubuntu Wiki, Ubuntu Forums and the Ask Ubuntu Stack Exchange site.
Pro Just works out of the box
Lots of support for hardware, lots of pre-installed software, and a smooth install process means less time downloading drivers, less time digging through configuration files, and less time deciding on software to use just to get up and running. It also means less time digging through forums looking for support.
Pro Polished UI
Ubuntu is designed to be used by everyday people. Because of that, Ubuntu has tried hard to make a user interface that's intuitive and looks pleasant and clean.
For developers this means working on the same OS as your servers, but with the benefits of a UI designed for humans.
Pro Use it on almost any device
Not just for laptops - Ubuntu was designed with tablets and touchscreen devices in mind, and with phone support on the way. Ubuntu also has Long Term Support releases, as well as a version oriented toward servers, so you can use the same OS at work or on mobile as you do on your desktop.
Pro Dedicated software center
The Ubuntu Software Center offers a GUI interface for installing new apps which is extremely easy and welcoming for beginners to Linux. But it should not be used by more advanced users since the method of installing through the terminal is much faster and easier after one is used to it.
Pro Frequent repository updates
Problems regularly arise with RHEL/CentOS because their repos are so old. One example is the outdated python packages required by YUM, which means we are forced to use a vent setup on all of those servers. In short, deployment times go from 2-3 hours on RHEL/CentOS to 5-10 minutes on Ubuntu.
Cons
Con Unity offers very limited customizability
The Unity desktop environment is practically non customizable with the user only being able to change certain themes and icons and adding a dock.
Con Tends to do its own thing
Ubuntu has a user interface that is specific to Ubuntu and is now working on its own display server. It also tends to have configuration that is done slightly different (and arguably more beginner friendly) than elsewhere.
So while this is a great Linux distribution in its own right, it might not be the first choice for someone wanting to learn Linux itself.
Con Canonical does not respect Ubuntu users' preferences
In 2012 it became impossible in Ubuntu to move the close-window-button back to the upper-right corner of the window, where it always was before. To the questions of their users Canonical replied that they know better than users where it would be convenient for users to have the close-window-button.
Con Relatively high system requirements
The default Unity desktop environment is a resource hog which requires hardware accelerated graphics rendering in order to run smoothly, making out of the box Ubuntu unsuitable for low end systems and older hardware. Even mildly aged hardware, you'll get far better performance out of a lighter desktop environment like LXDE or XFCE.
Con Using Unity locks you to Ubuntu
Contrary to almost all other interfaces, Unity only works reliably on Ubuntu.
Con Laggy default UI
Because of all the special effects there are in the default desktop environment, it is often slow in places.
Con Unstable
Some people pointed out that updating Arch is a high risk affair. And one should carefully read forums before doing it. The same is true about Ubuntu. Making system updates (like it was with 10.04 to 11.04) that screw so many things up became a routine. Even LTS releases should not give confidence that it will work.