The Vasari Corridor currently contains a self-portrait gallery of the artists who were courting the Court of the Medici for commissions. There are also modern artists' self-portraits recently added to the collection. However, if you visit the Corridor only for the art, you might very well be disappointed. Much of it pales in comparison to the masterpieces of the Uffizi Gallery itself, and many are suffering neglect and lack of climate control (a problem in many Italian art museums, even the Vatican Museum). It's a very dry presentation of art in the Corridor. Unlike the heat of the Uffizi, the Corridor is typically cool (and a bit damp)--a refreshing change if you've just come from trudging around the Uffizi.
When walking the Vasari Corridor, down below the streets will be filled with sweaty, shoving tourists (read, "commoners" or "illetterati"), while YOU will be like the Medici, strolling through your private covered walkway. Rain will never fall on your royal head. For the length of this exclusive tour you will become Medici. Your nose won't be bothered by the smells of the gutter--or cheap tourist food. You will be led by a Royal guard at the front and one at the rear of your very small tour group (so you don't try and pocket any art... so, OK, they don't really treat you like a Medici.) A warning if the tour group is a bit large... be careful about the many marble busts on columns throughout the Corridor. It is far to easy to back into one and knock it to the floor. In general though, tour groups tend to be less that 15 people or so and are well staggered. Touring the Corridor is an extreme contrast to visiting the Uffizi with it's heat and enormous crowds.
The real benefit is the unusual and unexpected views out the many windows along the Corridor. Views of the river, great architecture, looking down at the tourists below--all from very different vantage points--are the great photo opportunities that await. You can plan to visit the Uffizi first, then the Corridor (there are tours that give you both) and then come out into the Pitti Palace. As a bonus, the Boboli Gardens are right next door to Pitti. There are also combo tours which include the Corridor and Boboli Gardens. A great way to spend the day in Florence while experiencing both sides of the Arno River and four of Florence's best sites.