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?

yeah, I think we are learning immediate mode.  That's when every frame has to be re-drawn regardless of changes right?  I can't drop it, this is my last term, and if I drop it I'll have to take another term.  Thanks for the input though.  Is immediate mode never used in real-life?