MindManager is great, I use it for brainstorming and for organising all my projects at work. Top level branches are for projects, and branches off those are NAs for those projects.
However, this is the only part of my GTD system that isn't mobile and I can only update on my work PC (or from home over my VPN, but that can be too much hassle). But being able to see all the projects I have on, along with their NAs, makes it worth it for me. It prints really nicely too, so if I'm heading off to a meeting or something where I'm likely going to have to wait for others or have some spare time, I'll just hit "print" and grab it on the way out the door.