Not a good experience so far. Gave a location services error and said to enable it on device but didn't say whether that was in Watch Settings or Phone settings. Got that resolved.
Then said, "Open Google Maps" using Siri rather than trying to find the small icon for it. It says it can't find an app on the watch called, "Google Maps" even though it's on the watch and called exactly that.
I manually opened it instead. Little slow to load but not horrible. To note opening the Apple Maps takes significantly longer since it tries to open a map of current location. Google Maps watch allows you to click on home or work without using your phone. Where is to do the same with Apple Maps you need to use Siri or your Phone. However, Siri is far easier and quicker to say tap a button and say, Directions Home, rather than saying, Clicking through the buttons to get to the app and then clicking Home.
In regards to actually driving, it's relatively worthless compared to the very helpful Apple Maps. Apple maps automatically pushes to your phone each time you glance and highlights that particular turn as well as taptic feedback most importantly and feeds the audio through the watch. On the other hand, with Google Maps on watch you have to keep opening the app again, and it simply shows the list of directions. It does highlight the current step you are on, but far less attractively and the audio still comes out of the phone in your pocket.
All in all while hard to imagine anyone choosing to use Google Maps on Apple Watch over Apple Maps. And as data gets more and more accurate (as more and more millions of users use Apple Maps) the data will only get better and eventually Google Maps will only be used by Android users on the whole and desktop.