There are lots of apps that claim to be GTD. I've thoroughly tried every single one, Mac and PC. Some are, in fact, "GTD-compatible." But none actually DO GTD 100%.

I firmly believe that you have to have a system that works for you and THEN find ways to automate parts of it. I use a Google Calendar with SMS and email reminders in lieu of a physical tickler, for example. However, when it comes to maintaining project, waiting on, and next action lists, no program does it correctly.

The point is to get EVERYTHING into your trusted system so it's off your mind. If you can only get 90% into the program, then the remaining 10% is going to get pretty heavy, pretty fast. I'm an efficiency consultant and I help my clients use GTD to design an organizational system that provides them with FREEDOM, not restriction. You can only do this if you trust your system.

The main issue with the apps I've found is that they can't properly handle hierarchical tasks. Say you want to paint a room. You can pick the paint color OR measure the room first. You have to measure the room to calculate your paint needs. You have to calculate your paint needs AND pick the paint color before you buy the paint. You can buy the roller, painter's tape, and trays anytime, but you can't paint without them.

Try to get an app to reflect that like pen and paper can. (I'd LOVE to be proven wrong, btw.)