Big-O is an extremely easy way to sum up the running time of an algorithm. Sometimes it's misleading, but usually it's fairly informative, and with most code it

Big-O is an extremely easy way to sum up the running time of an algorithm. Sometimes it's misleading, but usually it's fairly informative, and with most code it can be worked out with just a glance or two. It's so simple and yet still reasonably informative that I'm not sure how it wouldn't be useful.