When Wingpanel Slim was introduced it brought a couple of problems: It replaced Wingpanel, making it rather complicated to switch back to the original Wingpanel, and it broke Galas Workspaces. These problems have been taken care of recently.
I have packaged Wingpanel Slim, so it doesn't collide with Wingpanel, and +Tom Beckmann has fixed the Gala Workspace issue. Additionally, elementary Tweaks now includes an option for Wingpanel Slim, making it easy to switch between the two panels.
InstallationYou will need to have the Community PPA activated, elementary Tweaks installed and an updated system for the Gala fix. Check this post for instructions on installing the Community PPA and elementary Tweaks. If you already have the Community PPA and elementary Tweaks installed, open up your Terminal (
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install wingpanel-slim
That's it! Switching to Wingpanel Slim and back to Wingpanel is easily done by opening up the elementary Tweaks plug from your System Settings and activating or deactivating the Wingpanel Slim option located under the Appearance tab. You might want to log out and back in for the Gala fix to take effect.