It all feels so natural, Kirk makes a surprisingly great father-figure to Craig, it makes wonderful progression for the character, and Beth and Kirk just work. I think it's because it wasn't engineered from the beginning, it all just came together gradually.
I do think this pairing has saved Kirk somewhat, he was getting to an age where it seemed he'd forever be alone. His long-time companions - Les, Fiz, Ches, had all moved on somehow and he'd stayed stuck in the same spot. I hope we see a long future for this new family, with Beth and Craig becoming Sutherlands down the line.