I used to use Evernote as my sole list manager. It's doable. I used the GTD setup guide available for purchase at the David Allen Company web site.
I never used Evernote tasks because they seemed to lack some of the fundamentals needed for GTD. The most important one was that they couldn't be easily assigned to a context. Yes, you could create a note for each context, but then you couldn't associate the tasks with a project note. In that case, I couldn't see an advantage over using another list manager.
Keep your eyes peeled for some major new developments with Evernote tasks, though. Bending Spoons is teasing some new functionality. It's too soon to say, but it's the sort of thing that could tempt me to use Evernote tasks for my GTD NA lists.