How long is a piece of string? How big's a unit?

I'm working on my monthly column for Software Test & Performance magazine, and I'd like your input. The topic, this time around, is unit testing (using Aspect-J or anything else). Care to share some of your hard-won knowledge with your peers?

In particular, what I'm looking for are experiences and advice about _developing_ the unit tests. (Managing and running them is something else again.) How big is a unit? How granular do you get? Do you have a particular process or checklist that you follow: ensuring that your tests look at interface, UI, etc.? Do you make a particular effort to create unit tests for the boundary conditions?

Anecdotes are, of course, enthusiastically encouraged. Tell me what you learned the hard way about unit testing. What mistakes have you seen others make? What would you teach someone about the process that might otherwise come only with experience?

On quoting you: My editors hate it when I quote "pookie-boy, whom I met on some newsgroup." Please let me know -- privately if necessary -- how I may refer to you in my article. (The usual format is "Kim Jones, a programmer at BigCompany, Inc. in Denver, Colorado.")

I'll be working on this over the next several days, so I look forward to hearing from you soon.

Esther Schindler
Contributing editor, Software Test & Performance magazine (stpmag.com)