I have tried out Google navigation on my Galaxy SII for the past three months and have found some of the directions both unusual and dangerous.
Example; grove Rd (Grand Canal) to St Brocs Cottages,(Donnybrook).
Left over Leeson st Bridge ... Make a U-turn! (Dangerous and illegal here).
From Morehampton road turn left down Mulberry Lane. (Lane unsuitable for driving).
On another trip I was told to turn right from Stephen's Green onto Merrion Row. There is no right turn here for years. In fact it is blocked by bus lane posts.
Last night I used Google to go to Cabra Castle in Cavan. It even suggested the address in the prompt. This made me feel confident. After a mad roundabout journey I ended up at some church 3 miles away from my intended destination.
On the route home from Cabra Castle I followed the road signs and the most direct route that I know. This differed considerably from Google Navigation.
The lesson here is ... Plan your route. Don't rely on Google. It will take you in the general direction, but may take you by a longer route. Watch out for silly directions which include U-Turns. I've encountered these quite often.
Google has turned into a monster of a company with no proper user support. Unlike Skobbler, there is no 'Bug Report' on Google Navigation, therefore they cannot benefit from driver knowledge.