Personally, I have used them loads of times and think they are great! They arent the be all and end all, its not an answer to all your problems in a tub...
I'm not convinced they are any good for cycling a tank on their own as such, I set up a 94L marine tank without them but using 50/50 mature/new water and the tank took just over a week to cycle of its own accord. As the tank was set up to try these things, I emptied it and started again with the Evolution Aqua pure balls in the filter and did the same 50/50 mature water and clean water mix and testing three times a day, you could watch the cycle happen a lot faster, took 3 days for the levels to reach 0,0,15 (ammonia, nitrite, nitrate) and putting in a little fish food to cause ammonia spike, it barely wobbled and ammonia never showed, only nitrates appeared within a couple of hours.
I also used them in my small tanks (Fluval Edge as freshwater and then marine), Fluval Chi and a cube tank, used them with each waterchange as it prevented any fluctuation when i did 60-70% water changes to keep them really clean and allow me to gravel clean in such small tanks.
I have also used them in ever canister filter i have used for the last year and a half/two years they have been around. Four on my own canister filters, one wet dry filter and loads scattered in various tanks, my cousins external and now a display tank external too. I do two 50-60% changes a week on the display tank and use the balls in the filter to keep everything stable. TBH I could empty all the water out the tank and do a thorough clean of the filter and using the Evo Aqua pure balls, I wouldnt see a flicker in water quality...
They work far better to help stability and prevention of problems than they do cycling (from scratch without mature media).
Work great in biOrbs as well!
/www.ebay.co.uk/itm/Pure-Aquarium-50-Balls-Crystal-Clear-Water-Bacteria-Filter-Balls-/320990798213?pt=UK_Pet_Supplies_Fish&hash=item4abc8ae185
