I fully enjoyed the pilot episode of Castle. Nathan and Stana have great chemistry, with him providing a ton of comedy as the crime fiction author to her straight laced cop. Castle is an engaging hero with a surprising amount of support from the characters around him, especially with his daughter and his mother.
I find the premise has tons of promise as the series progresses, both with the types of plots that can come from Castle's novels and with his romantic interests. The idea that a series of murders can be copied into real life seems a bit trite, but the execution was far better than I expected. The fact that Castle can keep up with Detective Beckett as far as crime scene analysis and motives is smart, but also a great source of comedy. Beckett's apparent infatuation with Castle's book series not only suggest that she's a potential love interest, but hints at a lot of character subtlety that has yet to come.
If crime drama crosses with romantic comedy sounds at all for you, then you should definitely watch and support Castle