Depends on their usage... In my opinion they don't belong on reef tanks. More suited for a fish only or other "high bio-load" setup.
They are an amazing and extremely efficient form of biological filtration. By nature of their design they house an INCREDIBLE amount of aerobic bacteria and can compensate for any ammonia or spikes of at breakneck speed. However this design can also prove disastrous.... Power outages or even disabling the filter for short lengths of time, say performing routine tank maintenance can quickly "crash" the filter. The mind-boggling amount of bacteria populating the sand once compacted and deprived of 02 can die, or will begin to die off immediately. Some one help me out here, hydrogen sulfide? Well anyway... fire this now dead and extremely toxic filter back up and well... badness abounds. Pro's and con's in a nutshell.
