What most people do not understand about melamine foam is it actually works as a very fine sand paper because of its very fine micro-porous structure. It is important to always use a liquid with melamine to provide lubrication. Melamine is very effective but it can damage some surfaces including flat paints and counter tops. As a result you will always want to test them in a non obvious location. Some of my secrets - they work great in showers with the fine white spots of soap scum that other things will not remove, they clean white sink basins like new, and they can remove a kool aid stain from a counter top. (Although melamine may remove the shiny finish so always get permission to take the risk first. I always say I can remove the spot but I may leave a dull spot and the customer almost always would rather have a dull spot than a red one.)
The other problem with these erasers is price. If you buy them at retail they can cost from $1.00 - $2.00 each. I buy imperfect (not perfect shapes) erasers from www.spongeoutlet.com. They work just as well as any brand and they cost only $0.33 each (with shipping) if you buy them in the case of 300 shown below.