Upgrade

That is totally up to you.

I bought one last month and upgrading was the first thing I did.

Do backup first.

If you are using iMovie HD 06 I would keep a Mavericks partition. There are certain issues with Yosemite and iMovie HD 06. (There are workarounds but keeping mavericks avoids this.)

Make a bootable clone of your Mavericks system. If possible, download the Mavericks installer from the app store and save a copy in case you need it. You can use Diskmaker X to create a bootable Mavericks installer on a flash drive. Do the same with the Yosemite installer. When you download the OS installer apps, you will see that they launch when the download finishes. When the first screen comes up, QUIT the installer, rather than clicking on "Continue". Save the installer app at that point, to a location outside of your Applications folder. These installers very unhelpfully delete themselves after they finish their installations.

You can install Yosemite on an external drive to test drive it. This is the least problematic way tp proceed with an OS upgrade. If you like it, you can clone it onto your main system drive using Carbon Copy Cloner. If you have no external drive, get one: it is an excellent safety net to have a second bootable hard drive, as opposed to only having a drive dedicated to Time Machine.

I would keep both Mavericks and Yosemite.