It's been my experience that people can't watch this show without breaking into a smile. It has the feel of authentic reality television--it's great fun to witness the players' moment of realization that they are on a game show, to observe their relationships with each other, and to see the host react to the situation. Surprising people with the possibility of free money makes for entertaining feel-good TV, even when the contestants don't respond with extreme enthusiasm (as they usually tend to do).
As far as the game show aspect, the audience becomes involved on many levels: not only can you guess the trivia answers at home, but because of the simple three-strikes model, you can compare your performance to the contestants, effectively judging how you would fare in the cab. And at the end of the ride, when the passengers are faced with the double-or-nothing choice, viewers can gauge whether they'll take the gamble. Whether or not you're familiar with Manhattan, Cash Cab is a delight. It's not a must-see show, but once you start watching, more often than not, you'll have trouble turning it off.