Traffic is never terrible, but there are some forced turns that can make travel on side streets frustrating for newcomers. The city is perfect for bicycle and pedestrian traffic, and cyclists are everywhere. Public transit is also quite good.
That said, speed limits in the city do not exceed 45, and are much lower on most streets, so don't expect to get through town quickly. Many intersections have cameras, so be sure to stop on yellow. Finally, there are numerous pedestrian crosswalks. Yield to pedestrians, or risk serious trouble.
Overall, Boulder is a beautiful place that is easy to navigate. Traffic is rarely heavy and usually polite.