I am only going to focus on Old Town as a dining and bar destination. It is many more things; boutique shops, art galleries, high end hotels, restaurants and even some multi family housing. It also includes the most high end mall in the state, Scottsdale Fashion Square.
Old Town is the nightlife center of the Valley of the Sun. I think this was best demonstrated when the Super Bowl was in town. The game may have been played in Glendale, but the celebrity parties were in Scottsdale. Over the past several years I have seen various celebrities out at the clubs in Old Town. I have seen our hometown celeb, David Spade, out over holiday weekends like Easter and Thanksgiving out in Old Town. Other celebs I've spotted out over the years are Charles Barkley, Steve Nash, Amare Stoudemire, Mike Tyson, Britney Spears, Paris Hilton, Kendra Wilkinson , the Kardashians, Dennis Rodman and Frankie Muniz (now living in the Valley.)
In March, during the Cactus League Spring Training, the bars are filled with groups of mostly guys that are in town to watch their fave teams play during the day and hit Old Town clubs and bars at night. You will also run into the occasional player or team scout.
The dining options are a mix of local and chain restaurants. The original PF Changs was actually located in Fashion Square Mall.