3/4 of the people I know are actively using it are on free tier. And based on all the HN conversations in the last year, plenty of HNers commenting here are also using free tier. I'd never go back to GPT-3.5, but apparently most people find it useful enough to the point they're reluctant to pay that $20/month.
As for the rest, OpenAI is apparently the fastest-growing service of all time ever, so that says something.