For me where Google Maps shines is in search. I was planning a trip which will include hitting the AKB48 Theater (won my lottery) and Google hits it no problem...iMap...no results.
Where Google Maps bombs and iMap shines is in local traffic. The Apple map nails estimated trip times consistently while Google Maps is always way off thanks to woefully outdated traffic data.
I use my cars' navi so I never bother with turn by turn. Looks nice, but not practical for me. I use phone maps solely for seeing when I should leave to get where I'm going or in this case planning a trip.
For now I am still needing both depending on what I'm doing.