Recs.
Updated
Pacman was developed to be the package manager for Arch Linux, and is also used by KDE-focused Chakra Linux.
Specs
Pros


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 Pacman hooks add power and flexibility
Hooks are scripts that can run automatically before and after Pacman transactions like installation and removal of packages. For example, Pacman hooks can be used to automatically create system users and files during the installation of packages. Pacman hooks can also be used to delete configuration files and clear caches during the removal of packages.
Pro Many wrappers available which extend functionality
Here are three examples: Powerpill, Yay, and Octopi. Powerpill extends Pacman by enabling asynchronous downloads of packages from multiple mirrors using Aria2 and Reflector. Yay extends Pacman, by enabling automatic download, compilation, and installation of packages from the AUR. Octopi extends Pacman with a GUI frontend written in Qt.
Pro Synchronizes package lists with the master server
It allows the system to always be up to date.
Cons
Con No official way to downgrade packages
This is related to it not supporting partial system upgrades. The very same reason prevents it from downgrading a particular package easily. Doing so can easily result in conflicts and may require a system downgrade or downgrade of multiple packages.
Con Doesn't always clean up after itself
Pacman leaves new configuration versions around and generally doesn't handle file conflicts smoothly.
Con Bloated
It is over complicated and bloated compared to other *NIX package managers like dpkg, rpm or pkgsrc. For example, they follow the Unix philosophy and do "one" thing well, which is to install local packages and use other frontends to configure, remote install or to search for packages/files. Pacman, however, tries to do all those tasks in one app, which adds some flaws.
Con No parallelization
Neither downloads nor processing can be set to run in parallel.
Recommendations
Comments
Flagged Pros + Cons
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 Simple package creation and distribution
pacman/PKGBUILD is easy to handle and yet powerful. Creating custom packages and distributing them via AUR is simpler than in other major distributions.
Con AUR packages have to be built from source
Arch Linux argues that, by storing all of their AUR (Arch User Repository) packages as source with a build script, they prevent unsecure packages from being uploaded to the repository. While this has been proven to be useful, it does make installing community packages rather slow.

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.
