I am a fan of feeders. It sad to me to see a fish beautifly designed for hunting and killing reduced to begging for food. I of course feed all my fish prepared food (most of the time) except for my knifefish. The highlight of his day is taking hours to hunt a big strong fish. Of course I don't just feed him "feeder fish" as these are bad nutrition, not to mention the thiaminaise problem. I breed platies and cichlid to feed him.
As for the diet balance issue brought up earlier, This is why i feed him my own fish. he eats big fat platies that I've raised in my own tanks feeding heaviliy with a balanced diet. This way I can make sure he gets proper nutrition. In addition i feed him shrimp and crustacions.
As for guppies, at least for my knifefish (he is a ck), they are his least favourite food. If i put many different kinds of fish in there, he eats SA tetras first, carp and larger livebearers second, and leave the guppies for last. I don't think he thinks they taste very good. Platies are better feeders IMO, they get bigger and have more babies