I read in Clickz a month ago that Google has plans to display ads (little blue pins) in their maps. I can't see these little blue pins (or markers) anymore in Google Local, but here it's a screenshot.
Maybe it was just a test and I hope so because I see some serious Usability issues in the actual behaviour of markers.
Where, how many, how to select?
Markers located very close superpose one over each other. As you can see in the image markers can be so hid behind others that you can't select them, you can't see exactly where they are located and you can't know how many they are. Sometimes there are so many markers that you even can't see the map.
When a marker is selected displays a globe with information. When you select one marker close to the borders of the map, the area of the map changes in order to make enough space to display the globe. This is confusing. You lose your location and need to drag the map to go back to where you were.
Using lists of items
It's not possible to know in advance what a marker represents. Does it represent a restaurant, a hotel or a bar? You need to select a marker in order to see what it is about. You also can remember the letter (A, B, C...) of the pin and check the list beside the map. Both systems are not very nice.
Google Local includes a list with the items represented by the markers next to the map. In a list you can easily find the information you need (name, telephone, price, etc.) faster than checking markers. It seems a good solution, but it's not perfect.
For instance, one of the main problems with the list of items is that when you have too many elements on it, you will need to paginate to show them all. In the map you only can show the markers from the items included in the page you are, but not the rest. That is what Google Local does at the moment and it is cofusing because the map can show no-marker where there is actually one, but in another page. Pagination is an element that works good with lists, but doesn't fit with user's mental model interacting with a map.
Google Local and API
Markers work ok after a search in Google Local because you know what to show and what area to show. You avoid some problems when there are few pins and the zoom is low enough. However pins don't scale well.
For sites that use Google Maps Api these issues can be more important. In some of them it makes sense to pan around the map or to zoom more than in Google Local. In these scenarios the usability issues of markers matter. Of course, sites can change the behaviour of pins for their own purposes, we already do it in Panoramio. However pins for ads are supposed to be defined by Google and not very much customizable. I really wonder how Google is going to solve these issues and which format will they finally use for ads in their maps.