Not sure about the apps. I just use GTD on paper using a paper agenda combined with a binder to store everything. The main benefit is not having to remember everything that has to be done. When I was younger I found it very unpleasant to have all of these things in my head all the time. Then I discovered GTD and never looked back. As remarked in the article, not everything that comes in gets done. Once every so often it is a good idea to purge tasks that have been in there for quite a bit of time and still did not get done.