Not a big time fan of Castle, but I do have to say I don't think there's a show on TV right now that's on as strong of an upswing.
The show's writers have found their tone. They've become very comfortable with Nathan Fillion's sense of humor -- something other efforts since Firefly haven't gotten right at all. While there's not a ton of chemistry between Stana Katic and Fillion, that seems to work to the shows advantage, since every show now feels the need to resolve the unresolved sexual tension.
It's still pretty much the last new episode I watch every week, but I'll give Castle the points for trying to ride the curve and become a much better show.