While Phos sponges work, they are only temporary fixes to the real problem you have created yourself by taking short cuts or going a cheaper way or adding things you shouldn't have. Phos guards and sponges only add more expense than doing it right the first time in the long run.
I have never used such a product, I find the cause, and stop it, theres no use putting bandaids on something that shouldn't be there in the first place if done correctly.
Or you can look at it like this, you don't want to buy a heater for your house because of the initial cost, but instead run 5 space heaters that drive your bill up incredibly high. Wheres the logic? besides, some phos sponges can leach back into your system adding to your problems.
Fix the problem if you can, don't cover it up.