Yes, you can really earn money with them, it's not a scam but reminds gambling strongly because as I know everything is based on their prices and they aren't stable, change really fast an unpredictably so every time you make operations with them you can various profit and sometimes even nothing at all
It also depends on a certain strategy you choose for making money from them, the most popular ones are trading & buying and mining. In case of first one you simply buy bitcoins for example for lower prices and then seel them for higher ones and mining is based on making calculations and building a blockchain. The second ones also demands rather expensive equipment for all that calculations and electiricty will cost you much too so it's better to have a good start capital before investing money in it. Also cloud mining is a popular strategy too, it's the same as classic one but in this case all that calculations are made by a certain company with which you sign up a contract, in this case your general profit can be higher. On the other hand fees for some companies are really high too and as I know there are not so many really safe companies in this case (sadly scam is widespread for them), according to serious resources about mining only such companies as Eobot, Hashflare
http://hashflarecode.com/ , Hashing 24 and Genesis Mining are really trustworthy.
Considering all these things I think that cryptocurrencies are a good additional resource for investing but becoming very rich with them is a rather rare thing.