I have been using the Walker iphone GTD app. Its great, but the horizons of focus part of it is really lacking. It feels like this part of the app was added as an after thought. The rest of the app is brilliant and I've used it very successfully. However, I am now feeling limited by the inability to be able to also track and monitor my horizons of focus.
'Horizons of focus' is a part of the GTD methodology of Dave Allen. It is a specific part of the methodology that deals with areas or scales of thought. The app I mention was great at everything GTD apart from that area. The functionality that would help in this area regarding that app would be if you could better organise your tasks by horizon. I wrote a review about it here: http://www.codebloc.co.uk/2014/01/best-iphone-to-do-list-app-uses-gtd-methodology/