The first season, while very funny and cool, does wear a bit by the end. It's not bad, ever, but by the end the cases have become predictable and a little cliché.
That sentence is all of the criticism I have, at all. For a 40-minute-long show it really does fly by each and every time. It's never boring and is pack full of great content. Information is never reiterated more than it needs to be, generally only having around 10 seconds at most, and two or three lines of dialogue. The viewer is never cheated because we never know more or less than the characters. We can't see the future and there is not 'stupid plot' (by which I mean a plot that only works if everyone is a total moron).
The series does do something different by having two very intelligent lead characters, and each character involved is also very intelligent, remarkably so. However, I don't think anyone would feel stupid by watching it; we always feel on the same level as Shawn and Gus, our fake psychic and cohort respectively. The acting is spectacular as well. In most comedy series there is a hefty amount of overacting and ridiculous characters (and sometimes that is used to great effect, just not often) but here there is a lot of serious acting mixed with accented underacting. It's very powerful and very fun.
Lastly, the characters feel very real. We can believe each character, even if they're a bit over the top from time to time, it's always what we expect. Shawn and Gus have many fun but real arguments, real life is never ignored. It's surprising how many real characters we run into. Sometimes they feel a little cheap, but never unrealistic.
We're only partway through the second season, but it only gets better. The second season is chock full of much more complex plots, without getting ridiculous. They have much more depth and the characters have more dimension. I'm excited, and I often can't wait to see what happens. Each episode only adds more awesome to the overall.