You know, I really have to hand it to this show. They've avoided all the other pitfalls most series have when they base the show around the love story of one couple. They didn't draw it out necessarily long (Bones, I'm looking at you), they didn't pull a Ross and Rachel and have the constant break up/back together thing, and going along with that, also avoided the usually extremely weak excuses writers will throw in to justify the break-ups and keep the "will they or won't they" thing going on.
They've actually naturally progressed their relationship, and I'm really enjoying this new outlet with Castle as a PI, it seemed like a natural progression for his character.
So while this show is formulaic, it's nice to see it's not falling into the traps that other similar shows have in the past.