OpenGL, is it useful?

I'm a class on OpenGL.  It's the only programming course on graphics I've ever taken.  How many companies actually use openGL to create software?  I've heard of OpenGL before this course, but it seems to me that DirectX is much more popular.  Is what I'm learning applicable to DirectX?

>>4
You'll find it useful to know that immediate mode is considered harmful.
http://eddieringle.com/ditching-immediate-mode-opengl-vertex-arrays/