I have bounced around a few different language classes now and found that there are some big differences in price, quality and atmosphere.
Depending on what you are looking for, you may not have so much choice. For example, when I was looking for an intensive week long course in the Summer, there was only one school offering the right course. If you are looking for evening classes though there should be a few different options for you.
Top choice from the schools I have been to is the Goethe Institut. Having talked to a lot of other learners, there is general agreement that even though it is expensive it is definitely worth it. The price seems to put off many of the less serious learners and it attracts the best teachers.
Another school that has a fairly good image is the Volkhochschule (VHS) but I always struggled to find the right course and the right time in the right part of the city.
If you are funding the study yourself, you should also consider going to a school that is elligible for state funding. In order to help integration and employment, the state will subsidise your study but only at certain schools. Take a look at Berlitz for more information.
There are lots of other schools in the city, and I have listed a few here for reference. These aren’t necessarily recommended, as I looked into them and decided against them: