I spend the whole day staring at light on dark text, but generally I find it difficult to read websites that have that setup. It's partly for the reason others have mentioned - changing from one to the other requires some time for my eyes to adjust - but it's aggravated by many sites because there is so much contrast between the text and the background.

I use a variant of Tomorrow Night, and the pastels on moody darks works well, but as things approach white on black, it leaves me needing a lie down