On the whole
Melanoides are a good thing. As Radar says, they keep the substrate nicely oxygenated, and this is especially valuable if you're using sand. They never harm plants, and while they might eat fish eggs, that's about the limit of the damage they can do.
Having said this, they multiply quickly. They turn any organic matter they find into new snails. It's worth mentioning that wild populations are reported to be measured in the
tens of thousands per square metre under some circumstances. They are also very long lived, over 5 years under lab conditions, and produce clutches of baby snails at monthly intervals.
Now, simply because you have thousands of snails in your tank doesn't mean you have a disaster, but some folks don't like to see them in such numbers. Once established, they're virtually impossible to eradicate. These snails are best controlled by keeping the tank clean, so that populations don't multiply too fast. The use of certain predators can also help; I find the snail-eating snail
Clea helena works particularly well. Some pufferfish and loaches will eat them too, if sufficiently hungry.
I keep
Melanoides in all my tanks and consider them extremely useful. But do your research first, because once you put them into your tank, you aren't going back!
Cheers, Neale