The city is old and has lots of history. The city center is close to everything and convenient. However, the city can do better to maintain the cleanness of the subway and train stations.
Bonn is not a spectacular metropolis like Cologne or Amsterdam, but it's a nice, youthful town with a lot to offer.
Even the nightlife is better than you would expect in a city this small, due to the high student population in town. And if there's still something this friendly and relaxed town fails to offer, you can always get to the neighboring city of Cologne in less than 30 minutes.