Recs.
Updated
Supported and stable on x86, x86-64, IA-64, PA-RISC, PowerPC, PowerPC 970, SPARC 64-bit, and DEC Alpha architectures. Supported, but not stable on MIPS, PS3 Cell Processor, System Z/s390, ARM, and SuperH.
SpecsUpdate
Pros
Pro USE flag system for extreme flexibility
A feature called USE flags control how packages are compiled from source, and what options are configured to get compiled. That means you can have per-packages USE flags to enable or disable certain features in them, or system-wide USE flags to enable or drop support for something you don't want entirely.
Pro Great for anyone who is serious about learning the intricacies of Linux
It's useful for both beginners and professionals. For the installation, Gentoo offers various types, which are referred to as stages. Basically meaning how in depth you would want to go into the process of installation. For beginners it's useful to choose for a starting distro due to its various stages that can be very time consuming but beneficial as you learn the composition in general of Linux.
Pro More fine-grained control over packages
Thanks to the Portage package manager, you can forbid the installation of certain packages by "masking" them, adding packages to different "world sets" for maintaining them separately, using stable and unstable branch of packages individually or system-wide and installing different versions of the same package in "slots". And unlike Arch, because of compiling, linker errors after package manager actions are less likely to happen, and when they do, you can build dependencies reversely.
Pro Vast possibilities and options for system programs
While having less official packages for desktop applications compared to something like Arch Linux, it has many options for system programs and utilities.
For example the choice of glibc, uclibc and musl.
Also the choice of kernel, vanilla kernel, gentoo patched kernel and even GNU/Hurd. You also have the choice to have a Gentoo FreeBSD.
Pro Good for people who want to compile everything
Based on everything being compiled from source which means it comes with a very flexible system for compiling packages - USE flags, CFLAGS, and company, both global and per package.
With USE flags you can select what is included in each package and with CFLAGS you can set various compiler optimization options (it's recommended to stick with the more common ones for most packages except those known to benefit from more extreme optimization as overdoing it can actually produce slower binaries, make compiles take forever and introduce weird bugs). Binary packages are available for some packages, especially those that are hard to compile correctly or those that take very long.
Cons
Con Customized package installation can take a long time and cause installation failures
The Gentoo package management system allows you to configure what compilation flags packages should support - i.e. specific processor flag support (SSE, SSE2, etc.), -O1, -O2, -O3 optimization, etc.
If you accept one of the default flags, Gentoo downloads binaries from the server. However, if you decide to optimise, it can and will download all source packages and start compiling ALL the programs and libraries on your system. If your chosen flags don't work with a particular library, installation will fail.
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Flagged Pros + Cons
Pro Live USB installation
Installation via a Live USB allows you to go back and fix mistakes from it without having to restart the whole installation process.
Pro Can be officially systemd-free
Gentoo's default init system is OpenRC. Gentoo also officially supports systemd-free Gnome and udev.