Working with GWT/GXT i like to speed-up my App with 'local-caching'. I red about HTML5 session storage, but i was wondering why i shouldn't just use a memory buffer (a big hashmap with all the incoming data).
Whats the pitfall with the memory buffer compared to session storage?
Just as what Thomas Broyer detailed in his comment, the pitfall for using a Map or any similar kind of data structure to save data is that all your data will be lost on page refresh.
If this is not a concern for your given scenario, I don't see any issue using Map/List or anything like that.
In the Errai framework we use a lot of @ApplicationScoped beans to hold data across the whole application, for example the currently logged in user, the latest loaded data from server etc.