Well, I've taken the plunge (ok, its not so much a plunge as we all know we can chop/change systems without too much issue), and am using GTDNext as my primary work list.
I too travel quite a lot and am genuinely struggling with the lack of mobile access but, for me, the outline style is working extremely well as many of my tasks are multi-levelled. The recent improvements in sequential tasks are also working well. I have previously used OmniFocus, NirvanaHQ and doit.im - and all of these have their own frustrations in the area of organising structured projects such as this. GTDNext appears to be the most suited to my workflow in this particular respect. This, for me personally, is that 'special' thing that @Folke alludes to in his post. Each person clearly has different priorities but this is an exciting one for me.
Admittedly I'm only in week 2 of this phase but so far so good. Doit.im remains lurking in my task bar (iphone and ipad too) but is now several days out of date. Yes, there are more features that are definitely required soon (mobile being obvious one, offline usage being a close second if mobile is too far out) but I am managing to work without those on the assumption they will be coming along in due course. If they don't come soon then I will need to re-assess but for the time being I am very happy with what I have seen so far.
The development rate is impressive and the interaction with the user base commendable. We have seen this with other 'startups' in the past who have failed to maintain that approach once they become more established, so lets hope James and co don't fall into that trap.
So far, so good for me. If you haven't tried it yet, I suggest you give it a go.