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SpecsUpdate
Pros
Pro Lots of 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 Dedicated software store
Ubuntu offers the Software Center in their older version of the OS as well as a program called Gnome Software, in their newest release of the OS. Both use a GUI interface for installing new software, which is extremely easy and welcoming for beginners to Linux. More advanced users can also install software through the terminal, as it tends to be much faster and easier, after one is used to it.
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 Company behind it
This means that the system must run well, otherwise they will lose money. There is no better incentive!
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 There are many different DEs with it's base other than Unity
Luckily Ubuntu offers full support for many branches such as Ubuntu-Gnome, Kubuntu, Xubuntu, Ubuntu-Mate, and Lubuntu. All of which come with a different DE than Unity. So flexibility is the word.
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.
Cons
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 Likes to reinvent the wheel
Cannonical tends to ignore existing solutions, tries to develop their own and in the end gives up. It is with their Mir display server and Unity desktop environment. Severe changes like that are not beginner-friendly.
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 Unity has relatively limited customization
Many users select Linux because they have heard it is customizable. Compared to other desktop environments, Unity has relatively few options to alter the look & feel.
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.
Con Custom PPAs required for most software
To get the most software available, user must add several PPAs to the system. That has major problems:
1) Terminal recommended for adding a PPA, which can scare away users
2) A PPA can potentially distribute malware by creating a "newer" version of a package than available in other PPAs, such as the Linux kernel.
3) PPAs must be recreated and re-added with every major system update.
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.
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Flagged Pros + Cons
Pro Great long term support release schedule (2 years)
This allows for users to always have a new supported release available without long unkown wait times in between.
Con Canonical (author) is proprietary
Canonical is focused on making money so you can see many proprietary offers by them and the OS has built-in trackers (Amazon).
Pro Has a clean Gnome interface
Gnome has lot of extensions available and can be customized rather easily.
Con Unity interface might be confusing for new users
Unity does not align itself with the normal desktop paradigm and thus has a bit of a confusing UI in comparison to the norm of a Windows user.