I don't like them. I think it is adverse to your livestock as it is not as natural an environment as you can get with an aquarium. Also, it keeps you from keeping a number of critters that you would be able to with sand.
There are some serious benefits if you don't like to clean or worry about decaying matter in the tank though. There is nothing to clean up and nothing gets buried in the sand and causes ammonia.
Others will chime in shortly and give their opinion. Some people really like bare bottom tanks.