I watched Castle for the first time last week. It was solid, but I decided to give it another shot. The result? Solid again. I think that the show did copy The Mentalist's formula of having a quirky non-cop with a hard-nosed female cop solving crimes together, but the material is still fresh on this show three seasons in whereas it is a little bit played out on that show.
The plot is also a little easier to follow, as evidenced by tonight's episode, on this ABC show.
I don't know if you can call me a Castle fan just yet, but I see why people have been fighting to keep this show on the air.