The Kronos is a must have tool for any serious racer, because you can never be sure how well you're travelling unless you're comparing yourself to the rest of the field.
This stopwatch makes it surprisingly easy to time 4 karts at once, and even includes the facility to time splits within the laps as well (though you need a sharp mind and fast fingers to do that for 4 karts all at once!).
Easy to use, this is a must. I have one in my pocket every time I go to the track, without exception.