Pros: Old Uppsala, real Viking mead, ambiance, old buildings, University.
Cons: Small, a little dead in the summer (students mostly gone).
Uppsala is Sweden's most prominent University City and has since 1164 been Sweden's ecclesiastical center. Uppsala University was founded in 1477 and is the oldest center of higher education in Scandinavia. Old Uppsala (Gamla Uppsala) also used to be the main pagan (Norse) centre of Sweden, and the Temple at Uppsala contained magnificent idols of the Æsir gods (Odin, Tor, Frej, etc). I studied at Uppsala University, I have relatives here and I have been back here with my family a number of times.
There are a number of interesting things that you can see in Uppsala, for example, Uppsala Castle built 1549, Uppsala Cathedral, which is Scandinavia's largest church, Uppsala University library (Carolina Redeviva), which hold 5 million volumes, and 60,000 manuscripts and is often said to be Europe's largest library. There are also many interesting University buildings, for example, Gustavianum built 1625. Gustavianum hosts the Victoria Museum, the Museum of Nordic Antiquities, and the University's cultural history collections. It also houses a perfectly preserved 17th-century Anatomical Theatre which was used for public dissections. Uppsala's city center contains many other University buildings, other older buildings, cobble stone streets, as well as a large number of café's. The old buildings, the café's and the river flowing through the city center (Fyris ån) gives Uppsala a special ambiance.
Old Uppsala (Gamla Uppsala) is located a few miles north east of Uppsala. In Old Uppsala you will find an Iron Age grave field with seven large royal mounds as well as 1,000 other archeological remains from the Iron Age and the Viking age. You can see a model of the Uppsala Norse Temple, visit an old 11th century cathedral, and drink mead brewed using a nearly 1,000 year old Viking recipe.
My wife thought Uppsala was a pretty neat small old city with a nice ambiance and filled with life and happy young people. I believe most foreign visitors feel the same way. Personally I love Uppsala, but I am biased. About a quarter of the inhabitants in Uppsala are students so Uppsala is a very young city with respect to the population. There are many café's, restaurants, discothèques, and clubs that cater to the younger crowd. Public transportation is fantastic and you don't need a car. Since Uppsala is relatively small you can visit most attractions in one day. Uppsala is only 43 miles north of Stockholm and there are frequently departing busses and trains that can take you to Uppsala from Stockholm.
If you are in the area on April 30th you should definitely visit Uppsala. April 30th is the Walpurgis festival and on that day a huge City feast is being held in Uppsala. For example, at 10 o'clock there is a float race in the river, and many of the students build creative contraptions that they float down the river. At one o'clock 50,000 youngsters are running down the main street and there are parties everywhere.
Thank you Chris for adding Uppsala