It's one of the best deals in fishkeeping: buy plants and get snails for free.
Seriously, they can become a problem if you overfeed your fish. I have them in my 75 gallon from the plants and have never had any problem. In the 29 gallon though, I became overrun with trumpet snails and have to remove all the the sand and replace it to get rid of them.
i had them in my 30 galon and they became a pest i had to tak everything out and clean everythgin if u wana know wat i did to get rid of them i can tell u b/c it worked and i no longer hav sanils its close to a month project though to make sure u dont have any and soak ur plants in a bleach and water bath if u wana try that leme know and ill tell u more i do that and so far no snails