Extremis is a lot of fun.  It takes a bold lunatic like Moffat to go for an episode being entirely in a simulation, but thanks to this being the start of a basically unheard of (at least in the Revival era) three-parter, it really doesn't feel like this story is without stakes or consequences either.  The stuff with the Pope at the start was very funny.  Moffat's sense of humour kind of comes and goes with how much it works for me, but sometimes, like here, I just find him hilarious in a very unique and specific sort of way.

So I imagine this episode is somewhat polarizing among fans.  While this trilogy kind of falls apart over time, I think Extremis works well in both setting things up but also standing on its own at least somewhat, making it a pretty entertaining if not that memorable of an episode.