Adding to Jefferson's reasons on "why Evernote" I would highlight that Evernote's search capabilities are amongst the most versatile. The setup introduced also relies on saved search strings to make GTD work in Evernote.
I've always been curious to try and make Evernote my GTD system. I use Omnifocus right now and it works incredibly well, but I also use Evernote to stash away all sorts of info I think might use later...sort of an external brain. Thus I rely heavily on Evernote's fantastic search functions but can't quite see using the evernote Mac app as my GTD Mac app. However, with Evernote based Swipes App that is coming to the Mac, and already in the iPad and iPhone, this might just be feasible now.