It is hard to give some specific information about German people. It depends on individual's own experiences and interactions with different people at different places. Generally Germans are nice people but you should be ready for some unexpected reactions as well. As regards the common Germans, they are nice but not very welcoming, some of them are not very comfortable with foreigners. Partly it is because of the mistakes made by the foreigners and partly because of the conservative nature of Germans.
On the other hand when you move in the educated lot and inhibitors of big cities you will find a better response. They are quite good and welcoming. One thing you must keep in mind while at University, in your department and with your colleagues; that don't wait for them to come and greet you. It's you who have to take the first step. Go to them, talk to them and once you will get to know them you will realize how nice and cooperative they are. Germans might now appear some what very cheerful and welcoming at first encounter. But once you built up a relationship with them you will discover that they are wonderful people. At times you get special treatment for being a foreigner and your colleagues go out of the way to help you. You may find friends and neighbors that are even better than you had before, however be ready to take the first step in building up a relationship.
You may also face problems in intermingling with other students when you register for a course. Usually the other students already know each other for some time so they have their own company. Further, they are also hesitant to come forward and talk to you because of various reasons like culture, language, shyness etc. You will have to some how break the ice and talk to them. It is important that you do so since you may have to do some project work as a team. Remember the principle that the path to heart goes through the stomach so you may use this formula here too :).
Generally speaking people are hesitant in speaking English. Not that they don't like it but because they are out of practice or have forgotten it. They can understand English but still they prefer replying in German. If you can speak a combination of English and German then they may also try the same. But some of them, like at the grocery stores, really cannot speak English at all. At big stores lyou will find people who can speak English but not at chain stores . If you want to have good social life in Germany then the key is to learn the language, there is no better choice. Once you will start speaking the language, you will find it much easier to intermingle with people.