I've been a Nathan Fillion fan ever since he had a bit part as the boyfriend on "Two Guys, A Girl, and a Pizza Place" (also the first big break for Ryan Reynolds, as well as Traylor Howard). I like how Joss Whedon showed loyalty to Fillion by finding him some post-"Firefly" work as a baddie on "Buffy" (and did the same for Gina Torres by putting her in "Angel" briefly).
Fillion's personality fits "Castle" well. His partner is just pretty enough and just snarky enough to be a good match. The actress playing his daughter is a gorgeous young girl and carries off the father-daughter relationship excellently, and his mother is also good (playing a warmer and less-drunken version of her character on "Dharma and Greg").
I thought it was hilarious when I was walking through a bookstore not too long after the show's creation and found a Nikki Heat book on sale, LOL. Authored by Richard Castle, with Fillion's pic on the back sleeve.