Before kiddos I took the apriori belief that it would kinda suck. The belief was unassailable because I thought, evolutionarily, if it was fun to have kids it wouldn't be fun to make them - otherwise we'd endure unfun "making" because we know the having would be fun.
And you were right.
Subjectively, having and raising kids is fun.
Objectively, it's not fun at all, but your mind convinces you that it is otherwise no-one would do it. This has been extensively studied across different demographics, cultures, age groups, evaluation methods used, etc, and the result is robust, i.e. consistent across all of them. Starting at a baseline life satisfaction level, it drops drastically when the baby arrives, slowly recovers a bit, then there's another big drop at age three, another recovery when they start school (but still not back to the baseline again), a huge drop as they become teenagers, and finally recovery back to the baseline when they leave home.
One thing I'm not aware of any work on is how the perception goes when the parents know about this in advance, a bit like being told how the magic trick works before it's shown to you.