They are nice to have. I mainly use mine for large batches of fish. they will cut your filleting time down cosiderably, once you get used to using one.
I have owned mine for 4 years, and filleted countless hundreds of fish from catfish and walleyes, to crappie panfish and trout, and have never needed a blade sharpened. I believe you can send the blades back to the manufacturer in most cases, but its not really needed.
I have a Rapala, AC/DC, and the knife is great. I dont like the AC box however, dosent run the blades nearly fast enough. Hook it to a battery though, it works wonderfully.
HB