Hi Lars,
I swear by the Lee Big Stopper. Very good filter as it preserves all wavelengths equally. It does add blue cast, so make sure your temp. is set to 10,000K (white balance).
It also adds a nice, natural saturation increase to foliage, blue sky etc.
You will also need a polarizer if you are shooting water, or sky.
The Lee polarizer can screw onto the front of the Big Stopper Holder.
Lee also now makes the Little Stopper, which I just got. This is ideal for lower light situations, when the Big Stopper would otherwise need, say, a 30 minutes exposure time (!).
Let me know how I can help further.