The USDA recommends cooking burgers to an internal temperature of 160F, at which point it will probably not have any pink in the center - but a burger cooked to 140F (medium rare) may also not have pink in the center! Color is not a reliable way to judge doneness. In fact, the USDA says 1 out of 4 burgers will be brown in the center well before they reach 160F. Insert an instant read meat thermometer horizontally until the tip is in the center of the burger. Not only is it an accurate way to measure, you won't need to cut into the meat (which lets the juices flow out).
Many people prefer their burgers less cooked, and that is a personal choice. It is true that the meat will likely be more flavorful and juicy when cooked to less that 160F, but as you decrease temperature, the risk of contracting a food borne illness increases. Young children, the elderly, pregnant women and any one with compromised health should not take any risk when it comes to these pathogens. According to Donald Schaffner, microbiologist at Rutgers Cook College for Food Science, about 1 in every 60-65 burgers contains a pathogenic E. coli cell, and some scientists estimate a healthy adult has about a 3% chance of getting sick from eating one pathogenic cell. Given those odds, you can choose to do as Dr. Schaffner does and eat your burgers cooked to 160F, or as I do and take the risk of eating a burger cooked to 145F.
Recipe: Sweet-and -Savory Burgers