Not every cloud mining site is a ponzi.Some are HYIP'S and some are sheer scams.There are a few trusted cloud mines though like the one here on the forum http://www.cloudmining.website/.There sig campaigns is been running since Jan 2015 so I'm guessing they're legit and safe to invest.More information could be found out on their official thread.
There is also a third party mining monitor, cmmonitor.com, which has reported to make ROI in ~114 days. Various reports in the forum indicates that there are large number of satisfied customers of www.CloudMining.website, though a few shill always try to distort the truth. Following screenshot is from cmmonitor.com, which depicts the truth...