Good info in this thread.
I have a pair of the wooden-handled Rapalas, one is really short and new, which is my main blade for trout nowadays. I don't fillet them, though, I just slice the belly, cut the heads off, pull the guts out, and clean the mudvein or whatever you call it. The longer one is a lot older, but same basic knife, and I'll use that for any filleting I do.
I already picked out my next one, which will be a folding Uncle Henry / Schrade / Taylor.
However, I'd like to get try my hand at making my own knives as yet another hobby. I don't see myself doing a fillet knife right off the bat, though. I think I'd enjoy the process, but we'll see. I lack a lot of the necessary tools right now.