I'm exercising GTD for a long time and I've always found it hard to define contexts for my work place. My role is a project manager and I work at the office where I have access to a phone, a computer, the internet at all times. I don't have any next actions to be done on the road. Some of my work I do from home (nothing that must be done only from home).
Due to the nature of my work, I rarely use contexts. The only useful context I have is @Agenda. I use it to maintain agenda lists with people I work with.
I'd be happy to hear ideas for contexts I can use so I can improve my productivity and better utilize my time and enhance my ability to choose the right next actions to work on.