It was pretty good at cutting down light and looking at crystalline structures. Next day I put a home made stop in my condenser tray for some oblique lighting and all of a sudden I got the results seen below (horrid pictures taken from my nasty tablet camera). I Then realized my stop was mounted on a cutout from a Starbucks plastic drink lid and it was acting as a birefringent (I think) and depending on how I rotated the eyepiece I had different polarization colors and background hue but no actual darkening.
Anyhow, it's an easy way to play with lighting and gives some very pleasing effects without the expense. The colors seen below are pretty close too.
20x sea detritus. Rubbish camera.
*Edit*
Just realized it looks like some ugly version of a Warhol
