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Arch Linux keeps things simple - it's lightweight, without the cruft of most other distributions, and completely flexible. These traits make it a popular distribution with many more experienced Linux users.
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Pros
Pro Fast and simple package management
Pacman has performance advantages over apt-get and yum in both database operations (thanks to being written for speed) and download times (by virtue of using better mirrors than other distributions tend to select by default). There are also fewer default repositories to download from, and all package management is combined into one tool instead of being split into dpkg, apt-get, and apt-cache like on Debian distros.
Pro As slim or copious as you want it
Arch keeps its core repositories slim and free of unnecessary dependencies. At first installation only a bare system is set up. You can easily get the other applications through the package manager. The repositories are nearly as full as those of Ubuntu, while they are often more up to date. That way you don't have to waste time with software you don't need or want.
Pro Excellent documentation
Arch's goal of simplicity means there's usually one preferred way to get things done - through organized and well documented configuration files. This focus, combined with the community's recognition that configuration files can be intimidating, has resulted in excellent documentation that's accessible to newcomers, and very instructive about how Linux actually works. The documentation is often so thorough that, when searching for solutions to problems while using other distributions, such as with video card drivers, oftentimes you'll find the most effective solution in the Arch Linux wiki or on the forums.
Cons
Con Rolling release requires bandwidth
Arch uses a rolling release model for updates. Unlike, for example Ubuntu where a new version is released every six months, packages are updated when they are ready. The advantage is a very up to date system and that the work of upgrading can be spread over a longer span of time to a point where it is hardly noticeably effort. However it can be difficult for people without a high bandwidth connection, or with limits on how many GB can be downloaded imposed by their internet service providers. A GB a month of downloads is quite possible.
This can be somewhat offset by Arch being lightweight, besides the relatively small core, the user selects what is installed (and has to be updated).