First of all, to get the user’s location, he must allow your site to have access to his location. But it is not always a good practice to ask for his location right away, it is necessary a "good reason" for such action. You may be reading more about it here: User Location.
Another important point, the Google Geolocation API, has a limit for FREE users, IE, make a test with your users, see if you will pass this limit daily, because if you pass, you will have to see if it is feasible to pay the API or even find another to supply the site. Example here: Google Maps Geolocation API.