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 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 Packaging signing keyring and mirror list
Both are also provided through packages which keeps them 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.
Pro Xz compression
Uses XZ compression for fast downloads & updates.
Pro Synchronizes package lists with the master server
It allows the system to always be up to date.
Pro Advanced when you want it
Pacman does not overwrite modified configuration files. Instead, it saves a .pacnew file for it which you can later merge at your leasure
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 Connecting to the AUR is a pain
For example, some AUR packages, used "git://" protocol to fetch code, may get blocked under restricted network such as behind a proxy, then you need to fix its build script by yourself.
Con No parallelization
Neither downloads nor processing can be set to run in parallel.