It all makes total sense!  He is just like Jane.  He is charming, manipulative, and arrogant.  Mashburn loved being a murder suspect in that past episode.  He insinuated himself into that investigation and played with Jane like a cat plays with a mouse.  He is intelligent.  He is abhorrently wealthy which gives him unbelievable power and resources.  His past romantic relationships were superficial and typical of men with narcissistic personality disorder.  He no-doubt considers his conquer of Theresa Lisbon to be another victory in his pocket.

I know that the episodes with Mashburn were played very light hearted and comically but I also think that his character could very easily turn evil.  I just haven't been able to think of a reason WHY Mashburn would be Red John.  Not enough history, I guess.

This also gives CBS a GREAT way to amp up the romantic angst for all the Jane/Lisbon shippers out there.  If Walter returns he will obviously try to seduce Lisbon and his vendetta for Jane will escalate to a murdering point.  Enter "Lisbon-In-Peril" scenario and Jane will have to outsmart a man who is literally as smart as he is.

I think judge Manchester is very likely to be red john...he easily has the power, money and influence for all this...also, something is seriously cookin in that poker group...
In season 1 episode 11, jared renfrew writes "he is man.." on the wall....dat could be manchester,...
also, in the same episode at the very end, red john calls jane and laughs....if we hear the laugh of manchester during the poker game in season 5 episode 14, its the SAME laugh....
One more thing...wen bosco was in charge of the red john case, red john kills him to give the case back to jane.....now in season 5, judge manchester effectively does the same when the case is almost taken by FBI....

I do not think Patrick Jane is Red John. BUT I do think that the writers have tried to point some evidence towards him being RJ just to manipulate the viewer. And they have succeeded, obviously.

The reason why I believe that in the end Jane is not going to be revealed to be RJ is simply because it's a tv show, not a (M.Night Shyamalan :p ) movie. That means that there's way too much material to cover at the end, when trying to tie all the knots together and prove that RJ and PJ are one and same person. To satisfy the hard-core vigilant fans, they would have to pretty much reshoot every scene in the series where PJ and RJ are interacting with one another, or where RJ's accomplices act nonchalant to PJ's presence to explain what has actually happened, and why they are acting like they are. And that would make the story line and ending way too messy and over-blown to be good tv. In a movie this would naturally work, because there'd be way less material  to cover in the ending revelation.

Another reason why I think PJ is not going to be revealed to be RJ is, that I think the Mentalist is  a classical "Superhero" story where Patrick Jane is the hero and Red John is the nemesis. Sort of like a Batman and Joker kind of a scenario. The story is enjoyable because it shows a hero "good" character slowly descending in to acts that are nearly equally evil as the nemesis' acts, in the name of catching the "bad" guy. I think this story line works better with the series.

I also believe that Patrick Jane does know Red John personally or at least knows something about Red John that he's not aware of. I think he's under hypnosis, and I think the mental institution where he was is the place where he first met RJ personally/was hypnotized. As PJ's IMDB Trivia page says, all the close-up scenes with PJ show his pupils dialated, which supports the theory of him being under hypnosis.

I do think that the creators have been toying with the idea of PJ being RJ, and in a way that would make a clever plot twist, though hard to execute. That would also explain why the RJ smiley face looks like PJ (c'mon, it does, doesn't it?). But I think it's more plausible that PJ is, without being aware of it himself, somehow helping RJ or feeding him information/being in close contact with him. And I think Lisbon is going to play a role shaking him out of the hypnosis (or whatever) in the end.

Thanks for taking the time to read.