Experts have said that the dive will be extremely dangerous. Although Baumgartner will be wearing a suit similar to those employed by astronauts, at 120,000 feet the slightest leak could cause Baumgartner's tissue to swell and the moisture in his eyes and mouth to boil. All that in addition to a potential loss in consciousness.
If Baumgartner's supersonic skydive is successful, he is expected to reach speeds of over 690 mph, becoming the first person to travel faster than the speed of sound without being inside of an aircraft. He will also break the longstanding record for highest free fall set by Joe Kittinger, a former Air Force pilot who free fell from 19.5 miles in 1960. Kittinger is a member of Baumgartner's team and is the only one who will be allowed to speak to Baumgartner during the attempt.
Although the jump has been delayed until later in the week (possibly Thursday), you can watch a dramatic CGI rendering of the feat below.