I think the only real similarity between "The Mentalist" and "Psych" is that the main character uses his hightened powers of observation to help solve crimes. This isn't a new premise (Sherlock Holmes, anyone?).
In "Psych", Shawn Spencer (played by James Roday) is passing himself off as a genuine psychic. From my understanding of "The Mentalist", Simon Baker's character, Patrick Jane, is a former stage mentalist -- I don't believe he's making any claims that he is a genuine psychic, however. Furthermore, "Psych" is a comedy, "The Mentalist" is a drama.
I've enjoyed "Psych", but I think there's also a place for "The Mentalist". I don't think that there's much similar between them, aside from the superficial plot device (which, as I mentioned, they share with the Sherlock Holmes stories).