Not a great one. The gag with all the similarly-named pizzerias was too silly. And I'm getting tired of the formulaic interrogations where they haul a suspect in, grill him because they think he's the murderer, and then have him give an airtight alibi that clears him in ten seconds. Why the hell don't they start
by asking for an alibi and save us all a lot of wasted time?
Plus, it was too easy to figure out who the killer was, and only partly because of the "the biggest-name guest star is the killer" rule. Liz Vassey's character claims she was attacked by the guy they're after, but nobody (including the audience) ever sees the alleged attacker? In a mystery story, if something is told but not shown, that's a pretty clear giveaway for misdirection.
And the weekly cell-phone commercials incorporated into the dialogue are getting a little too blatant. Castle actually says "There's an app for that?" Plus the whole "Murdered guy drops his cell phone down a sewer grate so that the cops will find it and see the crucial picture on it" plot point was a bit strained, like they were reaching to find a way to fit a cell phone into the mystery so they could spend more time talking about cell phones and their features.