I had always found with my 505 that if I held it at a pretty significant angle from my eye (as opposed to looking at it more straight-on), the background appeared much, much whiter than looking straight at it. The color I saw when holding at this angle was the perfect background color: a creamy sort of milk-white, not bright white like paper, but a very pleasing contrast to the color of the text. If eInk could reach this level of white background when looking straight at it, it would be awesome. As it is, everything I've read about Pearl to date, after boiling through all the effusive gushing over how wonderful it looks, seems to indicate that Pearl is really just the same as Vizplex with a darker text. As long as that background is that awful gray tone, eInk simply doesn't work for me. I can achieve the same results as Pearl with my 505 just by choosing a mid-weight or bold font.
Cream-white background with 300 pixels per inch or better: that's what it needs! I ran across an article from a year or so back that showed a demo eInk from Epson at a breathtaking 385 ppi! (Article here).
Sadly, that was a couple of years ago and I haven't heard any more about it since. The background on the demo still looked far, far too gray, but that pixel density on a huge display (something like 10.5 inches by 8 inches - every eInk PDF reader's dream!) would have really thrown the eInk industry to a whole new level.