I mostly use the Martitiini's marketted by Rapala for fillet knives. You can get a 4", 6" and 9" for the same price as the fallkniven. Flexible blades and work well for filleting fish. I use the 6" for trout , bass and walleye and the 4" mostly for yellow perch. I love how the leather sheath darkens with age, although one of my older ones is starting to get a bit smelly. Great little knives that do what they are supposed to do - clean fish. Handles are a bit too fat but the finger groove is nice. You can pick them up for $10-$20 a piece.
My wife also recently came home with a Wusthof 'Flexibel' fillet knife. 8 1/4 " blade. Nice handle slabs - some type of resin, thin but full tang. Feels great in the hand and has the required flex to allow cutting around, not cutting through, fish bones. I did a large rainbow trout and it worked really nice. No idea what the steel is on it but it sharpened well with crock sticks. Handle feels wonderful to hold. The markings on the blade say: Wusthof Gourmet, Solingen Germany 4618/20cm Flexibel. She received it as part of a cooking class on deboning and fish cleaning. The so called $100 value gift that came with the class - so I have no idea what its actual value is. At least they used the knife as part of the class. I still don't know why she took the class because she still makes me clean the fish every time.