The problem with the Red John episode is fans of the show care about the clues and want answers to certain questions, i.e. He is Mar, what happened to Kristina Frye, Rosalind Harker description of Red John, Sophie Millers description of Red John but Patrick Jane doesn't care, he found Red John and killed him, like he said he was going to do.

So the show couldn't reconcile what the fans wanted and what Jane wanted (or didn't want, for example he doesn't care how Red John knew his list of 7 suspects). 

So the show pulled itself apart in my opinion.

As far as Xander Berkeley, he's a good actor, he would have been a great Red John 10 years ago when he was playing George Mason on 24, Sophie Miller described RJ as middle aged, I think 57 is a bit old, mid 40's would have worked better for me, but here I am speaking as a fan, and Patrick Jane doesn't care how old RJ is.

I did like the way he killed RJ, could have lead to the moment a little better but I'll forgive it that.