Never changed from the default, so I haven't really thought about it much as I haven't seen a problem. Everything has been clear indoors and out, and my battery is still at 55% right now after a long day of moderate to heavy use. The default settings seem to be a happy medium.
I set mine to lowest when I was first setting it up. My battery life over the last three days has been great. And since I'm indoors most of the time, the lowest brightness has been enough. Curious as to what a brighter setting will do to the battery (well. I know it'll run down faster - how much is what I'm curious about).
I love how vivid the display and watch faces look with maximum brightness, but am not sure how much difference the maximum brightness vs. medium (default) brightness setting would affect battery life. Any ideas?
High, though I notice that within that setting it does auto adjust up and down a lot. It's noticeable if you hold the watch up to a light then look at the screen, it goes much brighter.
Don't think you can turn off the auto brightness though,
I went low because I didn't really see that it mattered while I was outside.
Funny story though: the first time I went out in full sunlight though, I looked at my watch and couldn't see a thing - and I was like "wow, that's kinda bad..."
But then I realized it hadn't turned on
I guess it depends on the face and colors you are using, too.