Have you ever had a really good employer? Whatever the answer, why was that the case? :)

Good or bad is difficult to say, it's kind of a gradient. And companies change all the time, so that was a matter of context, but I felt really good in companies whenever the people in charge were trusting me and the other employees.
As soon as the trust was not there, I always ended up leaving.
The second most important thing for me is to be in a company that encourages learning and self development.
After that, if your salary is right and you enjoy working with your colleagues, it's all in the details and it will be more a matter of taste I suppose.

I would say yes. I'm currently at the company in which I began my career three years ago and I consider it to be a great place to work. Now, don't get me wrong, there have definitely been moments/projects where I've questioned whether I wanted to continue there or not, completely due to workload.

In my opinion, determining whether an employer is "good" boils down to personal preference. Personally I like that my employer values time off by giving us unlimited PTO, trusts us and doesn't micromanage, allows us to work from home, and encourages us to speak up about things we disagree with. Every time that I, along with my coworkers, have felt overworked, my managers have held open discussions with us and we have figured out what the cause was.