The unequal subpixel sizes are due to OLED brightness fade; the blues wear out the fastest, while the reds last the longest:
Does it vary the intensity as it ages? Otherwise, it seems like the displays would start out way too blue before eventually becoming "accurate". (And then going on to become too red, of course.)
Edit: Oh, or maybe having a bigger blue subpixel means they can run the blues with less intensity, thus extending their life?
Don't the latest Samsung phones (S6) still use pentile? I don't think those look bad at all.
http://www.dansdata.com/images/cdv350/screenb560.jpg
The Apple Watch pixels are not in the traditional stripe pattern, but there's still 3 distinct subpixels for each pixel and they're arranged in a square pattern. The unequal subpixel sizes are due to OLED brightness fade; the blues wear out the fastest, while the reds last the longest:
https://en.wikipedia.org/wiki/OLED#Disadvantages