In my opinion, this is one of the better attractions in the Phoenix metro area. When people visit, this is definitely one of the places I take them to. There is a really large variety of activities to do here.
The main attraction is shopping. There are lots of great shopping places to go here. Then when you've expended all your energy having fun doing that, you can grab something to eat to replenish yourself at one of the surrounding restaurants. They have everything from good ol' cheap, good food to more expensive, higher-end dining.
BUT here is the things that Tempe Marketplace offers that other "malls" usually do not. There is a main stage where a band is playing almost every night (every weekend night at least). Plus, there are fire pits, a kid's water fountain/area and seating for people to just hang out.
The winter season is one of the best times to come here. There are usually lots of fun events. The past couple of years it will "snow" at 7pm. The fire pits are also a great place to socialize when it's chilly out. I usually like to grab a tea (from Tea Fusion) or a coffee (from Starbucks or Coffee Bean & Tea Leaf), sit next to a fire pit and listen to the live music.
Overall great place to go!