Pacman was developed to be the package manager for Arch Linux, and is also used by KDE-focused Chakra Linux.
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Pacman is objectively one of the fastest package managers around. This is because it's very minimalistic and it installs only prebuilt packages.
Pro Includes an easy to use build system
If the binary package that you are trying to install does not do what you want it to do, you can use Pacman's included build system to package and install it.
Pro Xz compression
Uses XZ compression for fast downloads & updates.
Pro A lot of available packages in Arch User Repository
The Arch User Repository (AUR) provides more packages for Pacman than any single repository for any other distribution. No more clumsy manual installations!
Pro Synchronizes package lists with the master server
It allows the system to always be up to date.
Pro Simple syntax
Pacman has a relatively simple syntax which is easy to get used to. For example:
pacman -Syu to update,
pacman -S foo to install etc...
Commands are not long and there are not many different commands to learn by heart.
Con Connecting to the AUR is a pain
Con Doesn't always clean up after itself
Pacman's simplicity isn't for the typical windows-level computer user because it leaves new configuration versions around and generally doesn't handle file conflicts smoothly.
Con No parallelization
Neither downloads nor processing can be set to run in parallel.