If you want to invest in cloud mining contracts you should not read only the nice successful stories you read about. There are many cloud mining sites which are nothing more that Ponzi schemes. I'm not talking about those. I'm talking about legit cloud mining companies which are having proven infrastructure they use for mining.
Before going to invest in a cloud mining contract you should do your own math, using a calculator to check the eventual earnings. I suggest one which accepts a difficulty increment parameter like ( cryptowizzard ethereum and zcash mining calculator). Then you need to look for the evolution of the difficulty in the last year. For example for Ethereum you can check it here.
What is difficulty?
Difficulty is a parameter in each proof of work blockchain platform which reflects the total computing power of the network. The higher more mining hardware is added to a network, the higher the computing power is and the higher difficulty gets. As a result the mined coins by each miner decreases(imagines the same amount of coins are split among multiple miners).
Now just insert your amount, pick a monthly difficulty increase of 20-30%(for Ethereum it went more than 100% for some months) and see which are your predicted returns. After that just go to see how much coins you can buy on the market, using the same amount of money. You can play with the numbers and try to calculate the returns for a 0 difficulty increase(which is quite sure it will not happen).
When should I buy a mining contract
I see only one scenario when you can do it. Let's say the price of a coin doubled or tripled in a few weeks and you are as sure as you can be that it's not going to get down too much. In that case, if the calculator predicts you're getting a break even period in 2-3 months than it might worth the investment. Otherwise it's better to go and buy directly the coins or to build your own mining hardware.
I personally made small profits using genesis mining(which in my opinion is the most legit, and as good as a cloud mining company can be). The best was a sum of around 90 usd invested in a Ethereum mining contract, which in about one year bring me 6 ethercoins. It looks like a really good investment, to have 1800 usd with 90 usd investment. It actually was but if I would have simply brought ethercoins with 90 usd at that point I would have around 40 ethercoins right now from that deal.