The "they are smarter than humans" thing is based on mere anecdote, not research. We can't know for sure that they AREN'T smarter than us, given that, intelligence and emotion wise, they are the closest to us :)
As for my own, personal belief, I think they are about equal to us. I've interacted with them many times, and they have such a human-like look in their eye, that I have never seen in any other animal (besides humans, of course.) It is amazing how they interact with us, like they are part of our own species.
I've spoken to a few dolphin trainers for Seaworld, and they are always talking about how the dolphins will pull tricks on them and then laugh about it!