It's fairly good, getting better. Best part is Fillion's acting, which makes it watchable beyond the somewhat lame premise.
Castle is not as good as Dollhouse, which may be the best show since Firefly, although I would also have to consider Rome, Deadwood, Galactica, and The Unit.
I'd trade them all for another season of Firefly.
Comparing other shows to Firefly is like asking who's the second best rock band in the 1970s: I dunno who was second best, but whoever it was, they were a long ways behind Led Zeppelin.