When I started to use my E90, I was a bit disappointed on the options, interface and possibilities offered by the calender app - certainly compared to the functionality I was used to on my old Psions. Howerver, after discovering SBSH Calendar, I must admit several of the flaws have been addressed, even though that good old Psion feeling of applications that can be used intuitively never returned.
In the mean time, time mgt concepts have evolved. I too adopted the GTD approach, and started to look at software that offered the flexibility and power to help me organise my life.
As a Mac user, I've tried Omnifocus and Things but finally selected The Hit List as the application that helps me focus on the things that need my attention. Until now I have been pleased with the choice.
However, in my view, a �real GTD system� anno 2010 would not be limited to a desktop app, but would need to be integrated with a smartphone. Sure, I can sync The Hit List => iCal, and subsequently iCal => SBSH Calendar, but neither iSync, nor the category approach and filters of SBSH Calendar are designed to offer the required flexibilty of viewing and selecting e.g. contexts, tags, next actions etc. that constitutes the core power of The Hit List.
I would expect my smartphone to offer the powerful features of full blown GTD software (so forget about software simply listing tasks) and in addition to offer additional benefits like e.g.:
*) integration of tasks and scheduling: if I schedule a meeting with e.g. Jack, I would appreciate my phone to be smart enough to select for me (and gently remind me) all tasks and issues I needed to discuss with Jack;
*) integration with GPS: based on where I am, my phone could (if I want to) remind me tasks I could complete in the vicinity of that location. Examples: buying the stuff on your shopping list(s) (because you're in the vicinity of a particular store), visiting that museum or expo you had noted as �something you wanted to do one day�, or talking to a person you needed to talk to and who happens to be in the neighbourhood, etc.
With all the functionality offered by modern smartphones, �context� based task mgt could be brought to a new level.