But it's all tradeoffs - in our app we have a few that aren't - for those, we profiled to find them, and added a bit of code to the functions to cache the return value.
Angular does dirty checking at the right moment.
After you bootstrap your app, nothing happens, unless: - user interacts (DOM event is fired) - response from server is back (xhr callback) - setTimeout fires - couple of other minor async stuff
So you only need to dirty check when some of these events occur and that's exactly what Angular does. Check out scope.$digest method, that's where all this stuff happens.
Polling the function for changes, contrast with how e.g. knockout.js pushes changes. (I could be wrong, please let me know if my understanding's not reality)