A context is a batch
I would like to chime in with the way I use Contexts. I view them as batching tools, irrespective of how the batch is defined. Let me explain...
One type of batching is the location-based context. I live in the middle of a town, with one child in a school on the Eastside, the other on the Westside. Since I am in those 2 locations almost daily, I have @Eastiside and @Westside contexts. For example, I have various shopping lists (Office Store, Hardware, Grocery, Costco, CompUSA, etc.) which I keep as #List-Office Store, etc., and I attach the physical context where the store is. (i.e. The office store is on the Eastisde, so I attach the corresponding context. However, there is a Grocery Store on the Eastside as well as on the Westside, so I attach both physical contexts to the #List-Grocery. This way, whatever physical context I am in, I know what to do.
Another type of context I use is activity batching by type. I like to do work in batches: i.e.: many calls at one time, hence the (tool-related) @Calls. However, I like to batch client-related activities, so I have an @Client Review context. Now, a NA like "Call Jim re: Follow up on my letter goes on both @Calls and @Client Review contexts, even @Office.
Although there is almost no barrier to switch from one tool to the next, I like the @Calls context because it is something I can get done in various conditions or locations, like driving, waiting for kids to get out of school, etc. (If you drive a lot, an @Driving context would be appropriate, but I don't do it often and long enough to deserve a Context in itself. Rather, while driving, I can think about the existing/available contexts and decide what I could do while driving -without becoming unsafe on the road, like calls, errands, etc.) Similarly, because while I am in the office I use the computer almost all the time and I am permanently connected tothe internet, I DO NOT have an @Computer or an @Online context. To me, it is indistinguishable from @Office.
To sum it up (in the hope that the above examples were helpful) I think that the most useful definition of Context is: "Something that I find myself doing often", which offers the opportunity to batch activities. What I found helpful is to keep the number of contexts relatively small (I have 14), but to attach AS MANY contexts as possible to each NA.
My PocketPC iPAQ allows me to sychronize multiple contexts to each task, so they are available to me in the office, or outside.
Calin