CAPTCHA is a divisive topic on HN, so you'll get a lot of people suggesting it's vital, and a lot saying it just isn't, depending on their own experiences.
"You probably don't need ReCAPTCHA" [0] is an article discussing techniques that has had decent discussion [1] on HN before.
I just want shout out a previous article published here in HN that sparkle a quite interesting conversations.
https://news.ycombinator.com/item?id=20158386
Long history short, don't use a captcha if you don't need it. And most of the time your website don't need it.