It's astounding how gullible tourists can be...and how much money they are willing to waste! To get scammed by (at best) sketchy hustlers to part with a great deal of cash, just to ride around Manhattan in a dirty open-air FUEL GUZZLING (read: MOST environmentally unfriendly) bus, driven by operators who violate traffic laws and create traffic havoc block by block, is not my idea of the best way to see New York. Apart from walking, which is the healthiest, cheapest and most interactive way to experience the city, visitors and residents alike can get the exact same tours for the $2 price of a MetroCard on the local city buses. The routes and destinations are exactly the same as the over-priced Grayline Tour buses, as roomy and comfortable (more or less), and most buses arrive every 10 to 30 minutes...and you can take all the pictures you want and hop on or hop off anywhere along the line that peaks your curiosity. Not sure of your bearings? Ask the driver (most are far more friendly than the tour bus operators). Not sure of a sight? Ask another passenger; contrary to popular belief, New Yorkers are more than willing to offer help, advice or recommendations. Never ever fails. For example, take the M6 local bus from 57th Street & 7th Avenue; travels right through the heart of Times Square, all the way down Broadway (past Herald, Madison, Union, Washington Square Parks) all the way to the tip of Lower Manhattan. Cost: $2 (MetroCard obtained at any subway station in the city; and exact change accepted as well on all buses!).
Two thumbs down to the Grayline (and shame on them)! If you can't walk the city, see the city the way the locals do...and save that money for a boat ride around the island or lunch at any number of fantastic eateries along the way. Or maybe a really good pair of walking shoes! Hope this helps.
Cons: damage to the environment, traffic violations, COST!, concern for safety, and on...