I prefer to use this developer undiluted. It gives a useful speed increase and a very smooth look -- I think it's great for portraits. I've used it with various films but find for me it's best with fast films such as Tri-X and HP5+ rated 640-800. That's not to say it doesn't work with slower emulsions such as Plux-X but I think it doesn't really get the best out of them because of the loss of acutance. You do need to take care with this dev as it can leave white stains on some surfaces and I recommend wearing gloves. This is definitely one of my favourite developers and I recently stocked up on it when I visited Berlin.