Moving on from the O6...
We know that, unlike Dave, Charlie has to be at least somewhat real, as one of the other mental patients saw him. As an aside, it was great that they didn't spoil this in the previews... I totally didn't expect it. For the whole time I expected it to be Dharma folks.
I like how Jack wasn't paying attention to the chase, until they mentioned the car model, and he got a perfect "oh crap" look on his face. Also, Jack pulling the trigger - awesome.
I love Mira Furlan. She needs to be a regular.
My favorite part of the episode though is that it's neither Jack nor John that people follow - it's Hurley. Hurley is the most charismatic person in the group, and this episode really capitalized on it. Everyone that goes with Locke goes because of Hurley. Like Rose said, if they didn't go with Locke, it's not because of Hurley... it's because Locke is a nutjob. What I found amusing is that most of the characters I would associate with a high Wisdom - Sayid, Rose, Sun - stay with Jack.
That said, I really don't get how they can justify Locke anymore, especially with regards to other people listening to him. While Jack has clearly gone off-kilter as well... I don't think that many people would be upset if he actually killed Locke.
My personal (crazy) theory about Charlie... I think that the "monster" is an interface to talk to the dead. Hence Charlie's line about being "dead, but here." All of the other instances we know of the "monster" manifesting have been of dead people - Eko's brother, for example. As to how it can manifest outside of the island, I'm not really sure.