they are ok if used with a stat they ferment too hot without one and could make your beer taste crap. Most people use aquarium heaters in a big bucket of water and then sit the fermenter in the water
Not good in my opinion, certainly very little control. The logic is that you can move the belt up the FV to reduce the heat. Given the exothermic reaction going on you'd have to constantly monitor it to keep the temp steady which isn't realistic.
I threw mine away and bought a £7 aquarium heater with thermostat incorporated.
Some stand their FV in a bucket of water and place the aquarium heater in that. I've cut a hole in my lid and dangle the sterilised heater in through the top into the brew.