Dusseldorf has two superb spectator Terraces and they have done a brilliant job in showing what can be done with a well thought out plan to provide views that are a match for any airport.
The deck at the main Terminal is very large and you have full access to both sides looking down on all of the air bridges, at the top end you find the best views for moving aircraft on either the inner taxiway or on the outer one which is well within reach of a 300mm lense even for the smaller ones.
The Terrace at the Railway station is much smaller than at the main Terminal but has a very close and lower view of aircraft taxiing left to right heading to runway 24 for departure with a 767 about 65mm for the camera.
Here in the UK viewing Terraces are poor to non existant and compared to the enlightened attitude of the German Authorities the B.A.A. make little effort to provide facilities and the ones that do exist are as far away from the action as possible.
In Scotland there are no viewing facilities at either Glasgow, with the Terrace having been closed in 1978, or Edinburgh where the one there was shut down in the late 1990's but where ones do exist elsewhere they are a struggle for the photographer.
Heathrow has a very small high Terrace which is distant from the taxiways and into the sun most of the day, it is free to enter due to the restricted views (good for spotters but not great for photographers) so I don't bother visiting it when I am there.
Gatwick also has a Terrace but personally I find it extremely poor with it being very pulled back from where you want to be and it is also high, again good for a spotter but dreadful for the photographer. I have always felt that this one should also be free to enter due to the restricted view.
So to sum up you will be hard pushed to find better facilities at any airport than what you
find at Dusseldorf, this is how the professionals do it and it attracts large numbers of people which makes the airport money from the fee to get in and there are security men patrolling the Terraces at all times to keep an eye on things.

This map above shows the layout of Dusseldorf Airport and where the two viewing Terraces are situated. Opening times are 9am to 8pm in the winter and 6am to 9pm in the summer with the change in hour taking place at the end of March and October.