It depends on for what purposes you need them and also how much money you can invest in general. Usually for the start it's recommended for newbies to have a start capital for getting more profit. Also everything depends on changes of prices for certain cryptocurrencies and they're very unpredictable and unstable, you'll need always to monitor them and control your operations for getting more money. Usually too popular strategies are used for this, simple buying and trading, and mining, however in case of the last one usually cloud mining as a more convinient upgrade of it is chosen That happens mostly because for saving money in case of expensive equipment and electricity, and there are a lot of special companies which solve these problems, you need just to sign up with them. But in this case only really safe and well-trusted ones like Hashing 24, Genesis Mining, Hashflare hashflarecode.com/ , Eobot and so on should be chosen because there's much scam for such companies and you can easily lose all your money. As I know one of my friends earned with various strategies smth like several thousands of dollars per month but he put really much energy in this.
So in my opinion cryptocurrencies are a good additional way of earning money but making them your only one object of investment won't make you really rich, especially in a long perspective.