(And remember: They're smarter than you...)
By one measure of intelligence. I've done pretty well in most standardized and IQ tests, and yet, in many instances, my intelligence is more analytical than anything else. When it comes to other areas, I know I am sorely lacking.
In contrast, I know people (e.g. musicians) who intuitively process music theory (or even math), people whose reflexes in sports are miles ahead of my own (to predict where a ball may be and how quickly you can intercept it is also a form of intelligence), and people who can read emotions extremely well and manipulate them.
So, yeah, while people who belong to Mensa may be smart in some areas, I would question the blanket statement that they are "smarter than you" (i.e. the general populace).