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The most popular Linux distribution, Ubuntu offers different OS versions for various devices like desktops, servers, tablets and phones. The desktop version, a fork of Debian, is very stable, particularly due to the number of active users, and provides a lot of features out of the box, so you can start working without having to install hundreds of drivers, tools and software first. For a lot of people, Ubuntu is the most competitive free OS to Windows and Mac.
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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 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 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 The rugged G series with the AMD Ryzen seven does not have support for the adjustable fan speed feature
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 Canonical (author) is proprietary
Canonical is focused on making money so you can see many propertiary offers by them and the OS has built in trackers (Amazon).
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.
UPDATE on 19.04 close button is back on top right corner
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 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 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 Unity and other Ubuntu-specific software is problematic
Unity is awesome and user-friendly at many places. But it's not customizable at much extent. Getting rid of it can be either not possible or very problematic. Same is true about other Ubuntu-specific software. A lot of things don't work as expected. Even though the repository is based on Debian, some ways to do stuff don't actually work. It's better usually to use GUI whenever possible, than to manually edit files. This is frustrating.
Recommendations
Comments
Flagged Pros + Cons
Pro Company behind it
This means that the system must run well, otherwise they will lose money. There is no better incentive!