Filters dust, covers exposed fan blades, cheap. Rip out the filter mesh and you have a decent fan cover that does not obstruct airflow as bad.
Weaknesses:
Gets dirty and must be removed to be cleaned. Removing can be a pain depending on where you put it.
Summary:
I have five of these in my 600T (only one with the filter mesh intact). I also have one covering a 120mm hole in the front of another case. They do what they are supposed to, but I wish ME would get in some other types of filters. Some are magnetic and easily removed for cleaning, these are not. But, hey, for $3 what do you expect?
Well built, looks like it would filter very well.
Completely seals fan off so nothing can touch the blades.
Weaknesses:
Attaching two (one each side) to a Thermalright X-Silent-120 fan pretty much reduces the airflow to zero. This fan doesn't start with much airflow to begin with, so it's not too surprising. I suppose only attaching the filter to one side would be OK.
Summary:
4/5 because it's well built and is useful for faster fans. The -1 is because it doesn't work well in silent fan environments.