I've found good and bad:

Good - I like having elevation profiles and accumulated elevation.

Bad - Doesn't always distinguish between paved/unpaved roads and occasionally will go out of its way to ride a short section of trail.

Hence, what I tend to do for Google Maps is:
- Instead work with auto instructions but with the "avoid highways" flag set
- Once I have the rough auto instructions, then perhaps create a bicycle route with enough way-points to see the elevation totals.

I don't mind riding some non-paved roads, but I'd like to have much better indication that this choice has been made. Otherwise, I might prefer an 18-mile all-paved route over a scenic 15 mile route that is half-gravel road.