Imo, in theory they’re great. In practice, in my experience with reddit, not so much.

You have a differing opinion? You usually get downvoted to hell and back. Without the downvote button, some people who would otherwise just downvote you and move on would have to engage you in a discussion. I think that’s a good thing.

Edit: there were rare times when a factually correct answer was downvoted as well which makes things confusing. It’s rare but it happens.