I lived close to Bremen for a few years. It's a nice city, pop. approx 1.0 million. I think it would be OK for a city break, if you've done the usual suspects already (Copenhagen, Amsterdam etc) and want to look somewhere different.
There are nice walks along the river Weser, and in summer there is a nice bar/resto scene along the banks of the river. You can visit the Becks brewery and have a few free pints. The city itself is nice too, and apparently the Cathedral is a big visitor attraction. Nightlife is not bad too, but Hamburg is better if you want more variety.
Bremen is a new Ryanair hub AFAIK, so I am sure they have some more info on their website about the city.