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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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.
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.
Pro Wide range of software out of the box
The default apps available in Ubuntu cover the gamut of most anyone's needs. From music, video or office applications Ubuntu has an app that will cover the users needs.
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.
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.
The setup for personal use is simple enough for anyone to achieve, and can easily be modified to act as a server. Programs and all features are easy to find and use, and first-timers can easily pick up on how to use it.
Pro Good UI for small monitors
Becaue the menu usually is in the title bar and the launcher auto-hides, the whole screen can be used by an application.
Pro Perfect for collaboration on open source projects
This is probably the best option when collaborating on open source projects in hardware. This is mostly because packages and tools are readily available via software center.
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 UTF-8 is the default character encoding
Beginning with Ubuntu 5.04, UTF-8 became the default character encoding, which allows for support of a variety of non-Roman scripts.
Pro Good PPA repositories available
PPA repositories allow you to install the latest version of your preferred software while keeping the rest of the operating system "stable".
Pro Unified search
Type any query into the Dash Home, and search will look through files, stores and web-pages to find what you are looking for.
Pro Cares about stable drivers
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 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 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.
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 Doesn't shutdown/standby properly on Lenovo laptops (b, e & g series)
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.
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.