Are they worth it or a waste of money?

 

I've never used them, but knowing how they work, I'd have to say they are pretty much a waste of money. They are called "f-mods" and are designed to filter the low-level signal before it goes into your amp.

BUT... unless it's designed to work specifically with your amp's imput impedance, the crossover frequency will be anybody's guess.

I would also wonder what effects it would have on the head unit. Although it's probably too minor to ever matter, capacitive and/or inductive loads put a much different strain on an amplifier than a purely resistive load, and the little head unit pre-amp may not be able to handle that load.