Cycling with a phone with the screen on and at full brightness (which is what you need on a sunny day), a phone will last a few hours at most. The magic of good GPSr units is that they use a transflective display, the sun is your 'backlight'. (And of course using something more akin to a microcontroller than a smartphone SoC.)

Also, in contrast to smartphones, these things are really rugged. Like last year I was cycling through the alps and accidentally dropped my GPSmap at ~30km/h (without a case or protection, who puts them on a GPSr?). It only has some scratches.

However, that combo is a battery hog. For some reason, OSMAnd drains a lot more battery when using it in guidance mode with Brouter, even though routing is much faster than with the built-in algorithm.

I heard that Locus maps has a much better brouter integration, though it is unfortunately closed-source: battery-efficient, automatic brouter detection, profile selection from within the app.

I also wish the brouter app would get a fresh coat of paint (a UI redesign), but that is secondary.