If you're around the area, Saigon Noodles is a safe bet for Pho. Some people love the place, but I'm not the biggest fan. The food is okay; nothing special in my opinion. However, the service is quite fast, and it's a great place for larger groups. The spring rolls are tasty, and they are made in an authentic fashion.
The portions are large, so you will not be left hungry. There are other places with better Asian-fare in Madison, but there are also places with pretty bad Asian food, so if you don't know where to go for your Vietnamese fix, Saigon Noodles is a good go-to.