My experience with 10.11 was also a very bad one.
However before upgrading I made a bootable clone external drive.
I found too many problems to keep 10.11, at least at this stage.
I therefore cloned back from the external to the internal drive and found in another thread in this forum the way to get back my Recovery Partition as well.
Any reasonable computer user makes backups before any important change.
If you did it then you can get back not only Yosemite but also all your Apps and data.
Otherwise you certainly can install back Yosemite if you had "bought" it in the App Store or if someone gives you the Installer... but how do you get quickly and easely back your Apps and your data ???