There are a lot of people using those controllers. As an electrician, I agree with Nam on this one. It isn't very safe. It probably won't cause a fire, I say probably because it is possible, but it will definitely shorten the life span of your motor. It will most likely overheat your motor and in all honesty it may or may not work that well depending on the motor you have. That is why you will have some people say they are crap and some that say they work great. It is all going to depend on how well your motor is made. It is going to basically chop your voltage input to the motor so the slower you turn the dial you will probably begin hearing the motor "bump" with each time it allows voltage to pass through. It is not a good way to vary the speed but it does work. Is it dangerous? It could be but probably won't be. There are a lot of safegaurds built in to UL listed devices so you will probably just smell a lot of smoke when the motor begins to fail but there is a very good possibility it could cause the motor to overheat when it fails and a possibility of fire. I don't want to alarm you and say every one of these is going to cause a fire because obviously they won't but there is always a possibility when using a motor for a purpose it wasn't intended to do. I would never use one but I understand the principal behind what it is doing.
Let me say this, if you are considering using it, pick up the phone and call the fan manufacturer's number and tell them what you want to use. Let them tell you how well it will work. I can almost guarantee the manufacturer will give you a resounding no. The motors on the fan simply weren't designed to be used that way. That being said, it probably will work. So, caveat emptor.