After seeing some very odd routing when I first started using it I set about fixing the maps on routes I was familiar with. After waiting a couple of weeks for my updates to go live (that's improved now) the routing was generally spot-on. You can map-edit within a mile of routes you've driven at world.waze.com
The trouble is - unless I've checked the maps myself I just don't trust Waze as a navigation tool. The automated map generation (or import, as has been used in the UK) doesn't include routing and the map needs to be driven or manually updated to populate junction rules etc. This means if you're in an area with no dedicated editors the map is the right shape but actual routing can be very ropey. There's also the possibility that anyone can go in and 'break' the map. Live on a main road and want to stop other Wazers being sent past your house? Just drive somewhere nearby, log in, block a junction or disconnect the road somewhere and job's a goodun. It's about as open to abuse as Wikipedia and I doubt errors will get picked up as quickly if you're off the beaten track.
I suspect some of your strange routing will be due to the way Waze uses traffic data. The routing uses historic data gathered from people running waze. Initially each road segment is just assigned an average speed but as more data is gathered it breaks each segment into half hour slots for the day. This leads to oddities such as it diverting you around stretches of main road in the middle of the night because it's only been used by other Wazers in rush hour. The realtime traffic then acts as an instantaneous over-ride to the general speed data. I gather that a minimum of 2 wazers are required to get stuck in a jam for it to automatically generate slow road speed data. It will sometimes ask you "are you in traffic" and hitting yes will generate a low speed but unless I'm stopped I don't really want to be asked to press anything. I generally refuse to hit the button on principle.
I can attest to the routing getting better as I drive routes regularly. It's diverted me around jams a couple of times and seems to give sensible routes most of the time. However, even where I've edited roads myself it will still pick a daft option. I've seen it route down a 'residents only' access road that I'd edited myself and set to private access in order to get around traffic. Other users have reported being sent incorrectly down a one-way street because the map is broken elsewhere.
I've stuck with Waze for a while due to its potential but I'm tending to use it less and less now. Waze sometimes gives traffic info for smaller roads that doesn't get picked up in Google navigation or Co-Pilot but this has never actually been of any benefit to me. I occasionally still use it for local routes when I'm expecting traffic as I'm either confident enough in the mapping to know it's not going to tell me to do anthing dangerous or familiar enough with the area for it not to be a problem if it gives a silly route. For anyone that just wants a reliable satnav app I can't recommend it.