I can only comment from a user's perspective but the issues I face with google maps directions:
- The warning to turn a few hundred meters out works, but I find in dense urban areas is that the "turn now" message is delivered too early sometimes. Where there isn't much of a gap between turns It can occasionally lead you to make a turn down the wrong (earlier) road. Low speed, heavy traffic situations are worst.
- It frequently leads you through backroads here (Jakarta, Indonesia), which often dont have much space for a single car. Allowances for developing cities need to be made given the width of roads. Gmaps has improved considerably in this area over the past few years but its still problematic.
- Heavy traffic estimations (eg where the traffic is a dark red on gmaps) are far too conservative for this and other similar asian cities. It's like it hits a theoretical max-time that a car can be queuing without taking into account cities with insanely bad traffic :P
- It may be annoying for some, but id appreciate a "last rest stop for xx km" message when on the highway if its above a certain distance (say 40km) :)
- Gmap specific: It's very unclear whether the navigation is on mute or not
