I think my only complaint with the 3XK thing (whose return, despite the fact I read most spoilers, I somehow managed to be surprised by) was the lack of Ryan's reaction to him being back. After all, he was technically the one that let 3XK escape the first time (since Castle wasn't even armed). Hopefully we'll get more Ryan the next time he reappears.
Also did anyone else notice how when Beckett was unloading on 3XK and he was still in the car she just had this fucking TERMINATOR FACE on. Like she looked positively murderous, no doubt because 3XK had spent the episode threatening/fucking with Castle's life. And then a few moments later he gets the same treatment from Castle because he was threatening Beckett. Moral of the story, don't fuck with Caskett.