I enjoyed the first series very much, was happy watching (but not so enthralled) with the 2nd series.
But have LOVED the 3rd. Most of the episodes are wonderful. Those that aren't, are still good.
It's superb family TV. Of course it is a bit formulaic, but what isn't that runs for more than a few episodes? There's still twists you don't see coming, and I think the actors are having fun. I've long been a fan of Nathan (since firefly). He's very enjoyable to watch and the chemistry with Stana is lovely.
Of course there is almost nothing original now about any new(ish) detective series and the 'they love each other but don't quite get together' tension has been used many times before. Yet Castle is still fresh and funny and on occasion very moving.