I was struggling with Contexts at first because I was creating too many or confused on where some actions should be filed.
I watched the following video linked from Omni's site. It taught me about using contexts in regards to time/energy/mood versus a specific tool for everything.
A Fresh Take on Contexts - Blog
http://simplicitybliss.com/a-fresh-take-on-contexts
A Fresh Take on Contexts - Video
http://www.omnigroup.com/video/a-fresh-take-on-contexts
A Fresh Take on Contexts - Follow-up
http://simplicitybliss.com/omnifocus-contexts-evolved