FWIW, I can understand the struggle you've had with contexts. I've been unsatisfied with my own for the past several months, and frustrated that I keep changing them.
I have found that location-based contexts work wonderfully when they apply. The @Errands is a great example. When you go out and about, it's a good idea to take care of all errand related actions at once. So, having all of these in one list by themselves works wonders (for me at least).
I wish the rest of my context lists were so clear. Since I work from home though, office/computer/home/etc. all merge together too much to be of use on separate lists, so I've tried variations. The latest seems to be working well though, and it includes contexts that are more about where I am in "mind" or "time of day" than anything else.
@Clients for instance, is a list of next actions that must be completed for clients. @Goals, is NAs that are up next for progressing goals. The others are @Home/Family, @ Other/Misc, and @TimeMatters. Time Matters is my desktop database and I use it for projects, contact management, email, documents, etc.
So far it's working nicely, but as with others I've tried... only time will tell