I've been frying eggs in my cast iron every morning for years, works like a charm! I use a small slice of butter and it's nonstick. The eggs don't freely slide around like they would in a non-stick pan, but after frying on one side for a moment I can easily move them with a metal spatula.

My trick though is that I have a cast iron only for eggs... if I cook other stuff in it the smooth buttery coating gets lost and the eggs start sticking again. After a few days of eggs-only it becomes nonstick again.