i use the caps lock key all the time (though you wouldn't know it reading my HN comments...)
any time i need to type more than 3 capital letters in a row, i hit caps lock. i type at 140+ wpm so holding down shift while typing reduces my speed by a significant amount. it's easier to hit caps lock and then hit it again when i'm done. it doesn't take any time.
examples of needing caps lock:
$LONG_VARIABLE_OR_CONST_NAMES
LABELS IN DIAGRAMS AND CODE:
3. CERTAIN FORMS LIKE PURCHASE ORDERS AND INVOICES ARE TRADITIONALLY IN ALL CAPS (QTY 2)
IMPORTANT NOTES IN MEMOS!!! to myself and others
i use it at least a half dozen times a day.