One of the best new shows on television this one has staying power. Great writing and a great cast, this one is a must catch.
The interaction between Beckett and Castle are what make this television worth watching. They bicker incessantly and fool each other at every turn but in working together can make incredible progress on their cases. By the second season, despite their continual denial, both trust each other - to the point that even when Castle interferes and inserts himself within an investigation, Beckett respects him enough to include him in interrogation.
Also, the focus on Castle's family life makes this particular show stand out among all those like it - we get to know his daughter and mother with the recurring storylines throughout the seasons. While it it is not the focus of the show, it provides a nice side-story to the specific case per episode.