I'm considering implementing a hybrid system...
I some ways, GTD isn't all that different from FC (FC has a "Master" task list similar to GTD's "Someday/Maybe"). I think that the main difference is that FC is more "Top Down" approach where GTD is definately "Bottom Up". GTD doesn't really emphasize setting priorities to your "tasks", either. I find that w/GTD, however, alot of my "QII" stuff falls by the wayside. I think that FC was a little better suited for me in that regard.
There are a lot of thing that GTD does wonderfully, such as collecting, processing, and filing - basically getting the stuff out of your head and in a place that it can be found when needed. It doesn't, however, really address the 50,000 ft stuff, and I think that this "omission" was intentional on David's part rather than a "flaw" in the system.
The main principle behind GTD is that if things are taking up space in your psychic "RAM", then your mental energy is spent thinking about what needs to be done. In order to free up your RAM, you need to empty your head and put those items in a "trusted" system. This, IMHO, is where GTD really shines. The bottom line is that it doesn't matter how the system reminds you (daily next actions, master list, prioritized, not prioritized, etc) as long as it's out of your head and you are reminded that "something" needs to be done.
I've been working with GTD for a little over a year, and the more I use it the more I realize that it is by design not a "complete" system (what I mean by "complete" is it doesn't have a "rigid" structure and a "set" way of doing everything). I think that that GTD is really a foundation that is mean to be built upon (I think that David even mentioned something to this effect in the book).
I now believe that my mistake has been looking for GTD to be completeley "strctured" (by nature, I am a VERY structured individual - just the way my brain works). I'm now starting to realize that the real beauty of this system is its lack of rigidity.
How's the old saying go - "don't throw the baby out with the bath water"? I really think that DayneB has the right idea; GTD can work best when it complements or completes your current system.
How do I handle the 50,000 ft stuff?
I try to get 50,000 off my mind, just like anything else. In order to do that, I just need to review what those things are as often as I need to, from that altitude, to ensure that I am doing what I need to be doing about it, or at least have actions toward it. I keep an area in my Palm Memos (a category called Focus Areas) with lists of 20,000, 30,0000, Blue Sky, and Principles. I also have an Inspiration category with all my personal affirmations, which handles a lot of my 50,000 focus. I don't review all those every week (don't need to usually, but just as often as I think I need to, to relax about what I'm doing and what I'm not doing).