[grigjd3]

I find Big O to be useful when designing solutions just to be able to quickly decide what data structures are appropriate for a task, though, admittedly, this doesn't come up that often.

[davidjnelson]

I find big o really helpful for designing back end caching algorithms and for optimizing sql queries. On a very rare occasion for spa ui as well.

[jniedrauer]

I do have a pretty good sense of how to structure things for performance, simply as a result of doing it over and over. But it's more akin to speed reading, where big O notation would be like sounding out each letter. It gives you the tools to explain to other people why something is better or worse.