Yesterday in the season premiere of Psych, Shawn expressed what the writers were thinking, essentially that The Mentalist is a carbon copy of Psych. Steve Franks, the Psych showrunner offered this analogy "When you go to the cereal aisle in a grocery store, and you see Fruit Loops there. If you look down on the bottom, there's something that looks just like Fruit Loops, and it's in a different bag, and it's called Fruity Loop-Os."
I have watched every episode of both shows and enjoy both of them, so I think I can provide an objective look at the show unlike the Psych writers who seem to have sour grapes about the commercial success of The Mentalist.
The premise of The Mentalist is a former fake-psychic turned police consultant. The premise of Psych is a fake-psych police consultant. That may be similar, but both come from the idea that if someone is extra-perceptive, he/she can glean information that others can't, giving the impression of psychic abilities. Since Jane in The Mentalist doesn't claim to be a psychic anymore, everyone knows he is just meticulous in examining things a trait in regular detectives of other procedurals. Both Shawn Spencer (Psych) and Patrick Jane (The Mentalist) say funny things, and have problems with authority.
The similarities between the shows stop there. Psych is almost purely a comedy. There are no laugh tracks, but a huge part of the show is dedicated to eliciting laughs. The writing is quick and witty with references to all sorts of things. On the other hand, The Mentalist at times delves into heavy drama though it can be lighthearted at other times. Patrick Jane has a past filled with anguish and he has a lot of issues because of it. Shawn is a classic slacker who fooled around his whole life. Psych's attempts at drama are in my opinion fairly weak and I don't know how they can improve that considering how over the top Shawn is.
Both operate with real members of law enforcement, but often, Shawn will do things on his own ignoring Lassiter or Juliet. They have never managed to stop him from doing something. In The Mentalist, Lisbon can put her foot down on Jane and stop him. Psych is dedicated solely to Shawn and Gus, but The Mentalist has a multitude of other characters to explore and a serial killer they are tracking.
Steve Franks got it wrong comparing them as Fruit Loops to Fruity Loop-Os. It's more Fruit Loops and Total. Completely different products with their own merits.