I got my PW a mere two days ago, but here's what I've noticed:
A bright setting in daylight will help with eliminating screen glare. This actually works in the same way that it does on a cellphone or any type of screen, really. On most sunny days I have to turn up the brightness on my phone because the glare caused by the sunlight can be quite intense and I won't be able to view the entire screen clearly otherwise. It usually isn't as big of a problem indoors, though. I find that a setting of 15 (+/-3) is usually enough indoors. I haven't tried reading outdoors yet, but I'm guessing that I would have to turn up the brightness to 20+.
In the evening, however, it's enough for me to read at a setting of around 5. Although that's with the nightstand lamp on. I haven't tried reading in total darkness, because, well.. I'm single and my dog doesn't care much about whether the lights are on or off.
Your eyes get used to the low light setting very quickly.