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 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 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 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.
Cons
Con Somewhat outdated solutions
While being outdated per se is virtually impossible for a rolling-release distro with a large community, a large portion of said community sticks to outdated solutions. For example, Gentoo's primary init system is OpenRC, which is cumbersome and awkward to use and provides little control over the system. While you can just choose systemd, it will require some tinkering. Other examples include stubbornly declaring an initramfs a last resort and an "oh my god 1337 H4XX0RZ surely have nothing better to do than trying for a month to exploit some vulnerability to steal my pony art, I have to fortify so hard my performance and ease of use will suffer" 90s security mentality.
Because of just how much freedom Gentoo provides you with, this usually isn't a big deal though.
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.