Just a note - I have a Macbook Air. I was running Mountain Lion and was totally happy with it, so when Mavericks came out I did not upgrade. I also did not upgrade to Yosemite until I became aware that El Capitan was coming out soon, so I didn't want to be 2 releases behind, so I took the leap. I had no issues with Yosemite.
I am now running El Capitan, and I am as happy with this OS X as I was with Mountain Lion and my brief time with Yosemite. I have had no issues.
Just one thing you want to make sure that you are aware of. With Yosemite, the iPhoto App was replaced as the default photo app. The default photo app now is Photos. When you upgrade to El Cap, your iPhoto Library will automatically be imported to the Photos app. Many people that rely heavily on how iPhoto (or Aperture) works, are not as happy with Photos. So, before you upgrade, you want to check your iPhoto version and make sure that it is a 9.x version. Once you upgrade, you can follow these instructions to get iPhoto up to version 9.6:
Get iPhoto 9.6.1 if you didn't update before OS... | Apple Support Communities
With iPhoto 9.6 installed, you will then have the option to continue to use iPhoto (or Aperture or both), or to use both the iPhoto/Aperture apps as well as the Photos app. I use all three. My import default photo app is iPhoto, but I also have Photo Stream and iCloud Photo Sharing turned on for Photos, so I can look at and play with my photos in ether app. Updates on one of the apps only updates the photos that are in that app's library.
You will also have the option to turn on iCloud Photo Library. Before you consider doing that, make sure that you are familiar with how it works and what it will change in the way that your photos work today. I have not turned it on because I like syncing only selected photos to my phone. I back up my Mac regularly, so I have a safe backup of all of my photos. I just personally have no interest in keeping my photos in any cloud service.
Here are a couple of articles that you will want to familiarize yourself with before you decide to use iCloud Photo Library. Be aware that this is a totally separate feature from the actual Photos app, so you can still use Photos without using iCloud Photo Library:
iCloud Photo Library FAQ - Apple Support
iCloud Photo Library Help - Apple Support
Also, before you upgrade, make sure you have a backup of your Mac.
Best of luck,
GB