jriihi, if you haven't seen an e-ink display for yourself, you really need to do that in order to evaluate it properly. I would not characterize the background as "dark gray" in any case, it is gray, but it's pretty light, and the background is a fairly consistent in all lighting.
That being said, the general rule of thumb is, the brighter the light the better it looks -- up until the light is too bright to stand, of course.
Of the reading situations you list, I wouldn't hesitate to use my Reader in any of them -- it's spectacular in sunlight, its great in regular indoor lighting, I don't know just how light your bus would be without sunlight, so I can't really evaluate that, but I can say that it works just fine on an airplane with the little built-in reading lights they have.
In the end, the best advice I can give you is my first advice: try to see one for yourself. Find a store, or a MobileReader near you who has one, and go take a gander at it. That's the only way you'll be able to tell for sure exactly how it looks to
your eyes. I'm pretty confident that you'll be amazed. I was
expecting to be amazed and I was
still amazed.