[brummm]

A lot of the comments below indicate that that the Du/Sie distinction is something old fashioned and young people don't do this anymore. This is arguably false. Even though it might not be at the same level as it was 70 or 80 years ago, this distinction is still very much part of the German language and society.

With people my age I would probably always use the informal Du, but only in informal situations. In case of formal interactions, as part of a business transaction, etc. I would probably always go to the Sie if I didn't know the person and would expect the same in reverse. Anything else would probably be perceived as lacking respect. English doesn't have anything equivalent except maybe first name/last name basis, so it's always hard to explain to Americans.

Of course, as a foreigner things are always less strict and one gets some slack for not speaking the language perfectly.

[t0mas88]

It's a cultural thing that's subject to change. The Netherlands technically has two forms as well, but the use of the formal one in practice has greatly reduced and people don't put much weight on the distinction anymore. Certainly wouldn't get offended if you used the informal one. Used to be different 50 years ago.