Surprise: I really liked it. There was at least a brief hint of that old Key magic in Mio's last "confession" re: Midori and her request to Riki before vanishing. After that it got a little muddier, but I still liked it more than the derpy girl's arc. I really hope this is a sign of things to come, and not a high water mark.
Also, currently, I ship Riki with Mio... though I think she ships him with Kyosuke.
It's hard to tell exactly what's real and what's not. As we only briefly see Mio and Midori in the same place at one time, and Midori is an aspect of Mio's own self, it's theoretically possible that much of the supernatural element can be explained away if Riki's perception of events is shown to be unreliable. I did like that they explained Mio's relationship with Midori by showing how she had an aspect of herself that she'd externalized medicated away by well-meaning doctors, and that's what caused her "incomplete" state. I like that better than her just becoming "invisible" because she's been so long ignored by classmates.