Also, I think that a proper todo app, in addition to geofencing, should include timefencing, people-proximity-fencing, external-condition-fencing and maybe even mood-fencing / energy-fencing. Though I guess some of these can be implemented via contexts.
I grudgingly switched to Asana a few months ago and haven't looked back. It supports a GTD workflow with the addition of excellent communication and integration with other tools. For example, I automate task creation with Zapier to follow-up on calendar events in my diary, follow up website leads, scanned biz cards, etc. Placing tasks in multiple projects is also super useful. Omnifocus still has a better mobile app UX, and I wish it would offline sync to make better use of precious planning time on my commute.
The part of this post that I already found useful in the few hours since I read it is adopting the Pomodoro time management technique. I found a neat OS X app that called PomoDone (http://pomodoneapp.com/) that syncs with Asana and many more task managers to keep focus in 25 min intervals followed by 5 min breaks. Thanks!
As I've grown from lead developer on a 3-person team to CTO of a 40-person company I've had to get more serious about my GTD practice. Where once I had several inboxes: OmniFocus for personal/recurring tasks, then Gmail, Pivotal, GitHub and Trello, now I've doubled down on OmniFocus as the source of truth for my workload. That doesn't mean projects are managed there, only my personal tasks related to the project that will take longer than 2 minutes make it there. Chucking things into OF from many different sources becomes very easy with Quick Entry and Clippings tools, and it is invaluable for planning each day and making sure things don't slip through the cracks. Typically I'm collecting things in OF all day and flagging important things, but at the beginning of the day I actually set a Focus on work projects, and write out in my notebook the list of priorities for that day. I know GTD is all about getting away from re-writing lists, but I find it helpful to have a hard copy of my foci sitting in front of me that doesn't take up screen real estate is impervious to the daily influx of distractions.
Basically, if you tell siri to remind you of something, it creates a task in the Reminders app. OF imports this, including due dates.