Yosemite, good thing? bad thing?

My laptop is working fine with version 10.9.5 however 'update' keeps prompting me to install Yosemite.

In discussions, I have seen that folks have problems after Yosemite is installed. Why does update keep prompting me to do this?

What is the downside of not installing Yosemite? How do I get 'update' to stop prompting me to install Yosemite if it's just going to cause problems?

Thank you in advance for any insight on this!