This question is a bit biased towards the pro crowd but it should be an enlightening informal poll for everyone.
What have been your favorite touch up stones to use in the workplace? Do you value an aggressive stone or something of an extremely high grit? Do you go for good value stones so if it gets broken you don't have to cry over the shattered remnants (stories welcome), or do you find its worth the risk for a better stone? Any particular brands/styles? Limit yourself to just one? Portability?