Most CAPTCHA's nowadays I find unnecessarily complex. Use tricks like timing form completion (<50ms? Bot), hidden fields, etc, before ruining the UX with CAPTCHA.

Then again... does HN really care about UX? Token expiration after x time when browsing through the listings, ancient unresponsive design, etc.

There comes a point where it'll be more cost effective for spammers to just farm out the solving of CAPTCHA's to people in third-world countries. It just depends if there is enough value in spamming HN for them to bother (probably not, given the user-curated-and-rated content model.