I'm OK with this
Honestly, I'd rather give up CPU cycles than see intrusive (or any, really) ads. I'm fortunate enough, probably like most people here, to have a powerful CPU, so the cycles are spare anyway. I could see how for a lot of users it'd be more of a problem.
It'd be nice to see it evolve into a way for it to be managed, via some sort of side action of seeing how well the page is loading, to determine how much strain the mining is having on the user.
But when I'm browsing the web, it tends to be all I'm doing (bar a tv show or movie playing in the corner) so I think it's a great way to help pay for sites.
The problem is the internet has always been "free", and that mentality is never really going to go away. Far too many people see ads as some sort of insult almost, even if they're not intrusive; just them existing is a problem. But content creators need to get paid!