Recs.
Updated
Pantheon is a lightweight and modular environment written in Vala and GTK+. It's the default desktop environment of elementary OS.
SpecsUpdate
Pros
Pro Splitting and animations
Switching between workspaces is fluid, pulling up the workspace preview bumps up the rest of the desktop, open windows show up on the switcher along with larger icons, and everything is subtly animated. You have two windows taking each half the screen by dragging to corners.
Pro Slick, smooth desktop environment
Easy to use and easy to configure, there is almost nothing that the developers did not think of to creat a top-notch user experience.
Cons
Con Few configuration options
It is pretty bare-bones on a stock install, but further configuration options can be added through various official tweak tools.
Con Not ideal for enterprise environments
As it has no set release date, and no set support and/or EOL dates, that makes it a not so good choice for enterprises managing a number of machines where such dates are important to know before hand.
Con Limited to eOS-only
It means Pantheon won't work properly if installed on other distros. Manjaro has given up on trying since it works better on a LTS system and takes an eternity to realease a new version or add security updates. On Arch, Pantheon also doesn't work properly. People are trying it on Fedora, not sure how it works now. If you want to use Pantheon, better get to eOS.
Con Looks and feels like a Mac
Which may turn quite a few people off with the aesthetics, of course for some people, this is a pro.
Recommendations
Comments
Out of Date Pros + Cons
Con Lack of portability
Currently, as of end 2014, Pantheon is working well in Elementary OS but installing it in most distribution will be a manual process requiring some tuning and fixes.