A productivity system that doesn't make you feel good about your daily life is not be sustainable in the long run. It doesn't matter how effective it is, that's not a life you want to be living.
What makes you feel better about your daily life will depend on your temperament. The author of this blog post found that GTD resulted in his feeling like he had a never-ending river of stress ahead of him. GTD failed for him.
Another example where GTD fails is for anyone who is doing sustained creative work. As Paul Graham describes in http://www.paulgraham.com/makersschedule.html, it is very important for those people to have large blocks of time set aside for focused work. GTD's focus on breaking things into simple tasks whose execution is routine is the antithesis of how work needs to be organized for a maker.
That said, GTD is life-changing for many people. If it works for you, by all means do it. But don't make the mistake of trying to convert everyone else to it. It isn't always right.