Flatline is rad as hell.  It's a very fun episode where Clara gets to take centre stage in a way that felt both natural and much-wanted.  I wish I liked her as a companion more.  Jenna Coleman is clearly an incredibly talented actor, but often this show felt like it had very little for her to do or even enough space for her to be a real character other than just as a sounding board for the Doctor.

Yet here she gets to take the lead in solving this mystery as a small part of London becomes terrorized by the Boneless.  She really drives the story forward in a cool way, using the sonic and leading the other survivors against these monsters.  It's brilliant and not only one of her better episodes, but also one of the better episodes in general to really rely on its companion characters.

The Boneless that confront Clara and the Doctor are mysterious in the best possible way at the start of this episode.  I like how their nature as extra-dimensional invaders is expressed in them being essentially flat.  It made for a lot of creepy scenes and the Boneless felt very powerful, especially as they were able to affect even the TARDIS.

One thing that bothers me, and it is mostly minor, is that the Boneless are only named at the very end.  It might just be my Canadian ears, but when the Doctor finally names them, a big moment the whole episode feels like it is building up to - I actually can barely make out him saying "Boneless."  Maybe it's Capaldi's accent, maybe it's just the sound mixing of his line is too low against the big sound effects used in that moment, but either way it blunts the hell out of that scene's effectiveness.  It's a great episode overall though.  I like the characters who were all pretty distinct.  Rigsy was a fun companion of sorts for Clara, and the cop at the beginning was good too - what happens to her is quite brutal for Doctor Who standards and it gets into the episode's more distinct and memorable qualities - the Boneless are true menaces that feel dangerous and scary - I love it!  That one supervisor guy was a little annoying, though.  He felt unnecessarily antagonistic and blemishes and otherwise excellent episode.  I definitely enjoy revisiting this one, too.  Top-tier stuff overall.