Eyes open or closed?

Depends. They're always open when I'm driving, for example.

Open, and with good imaging I can see the performers. Especially with a black background.

Closed eyes I'd be asleep in no time. 

I generally listen with my eyes closed.  @cleeds exception noted.  I like to daydream when listening to music.  OK, sometimes I nod off, but generally speaking, If I'm sitting listening to music with my eyes open, I get distracted from the music.  It's much easier to slow my mind down and get immersed in the music with my eyes closed.  

Closed.  Every time. Easier to lose the speakers and become immersed in the soundstage. Everything sounds better with eyes closed, and it costs nothing.

Eyes opened (reading) for a while before going into sleep mode...

+1 @gdaddy1 

When your system throws great images, no point is not watching them dance in front of you. That is the actual fun!

Usually, eyes opened in a lighted, or sometimes, a dimmed, but not dark room. 

For me, the image and soundstage on my system, is so well defined, closing my eyes does not help enhance the illusion at all.