His fame gives him an all-access pass to her cases and he's working her last nerve. But somehow, together, they're the perfect partners in crime.
Castle, the title character, is a master of snappy dialogue. Detective Beckett keeps things serious and grounded, but still brings an element of fun and humor when she lets herself. The "sidekick" archetype gets filled by a pair of "buddy" cops in the department, who still always manage to prove themselves competant detectives.
It strikes a nice balance between "murder of the week" and a few long-running plots and adversaries. The chemistry between the two series stars throughout their partnership keeps the will-they/won't-they romantic relationship interesting.
And if you're a Firefly fan, there are enough subtle references (and some not-so-subtle) and guest stars to remind you that Nathan Fillion had a life before Castle.
This show was a delightful surprise.