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?

>>13
OpenGL is just graphics, it doesnt create any windows or any other platform gui widgets. So I just want to know what gui toolkit is good to use with OpenGL