I was impressed with the viewing area at DUS when I went through there in summer 2007. On more recent trips to Germany I have also had the chance to spend time at the viewing areas at BRE (Bremen) and HHN (Hahn) which also offer excellent views of the apron/runway activity, although Hahn is a little limited to Ryanair and some fairly distant cargo airliners and is behind Glass, it's still a nice addition. Bremen is outdoors however, has a museum and also offers a more varied range of aircraft and airlines.
I definately think the UK and other countries should follow suit with German airports and viewing facilities. At BRE and DUS they were quite busy with passengers just passing time, of course there is little/no money to be made from them as there is no room to fill them with shops and bars and many people would probably not pay a lot to use them (although DUS charges and was still popular) which may not be of much interest to many airport operators these days.
