So many cloud mining scams! So many promises! Having gotten (mildly) boned by that miningunited scam last year, I know the feels.
Post the tx ID of your payment to them so we can all see it on the blockchain (better proof of scamminess) and move on to pastures new :/
Cex.io is plenty legit though. I and about ten friends had a bunch of payouts (min 0.01BTC) from them with no problems, they refund downtime, and have been pretty cool when I've poked their support.
No preorder scammery since their cloud Ghs are available now, and they mergemine, so you get a lump of spare Namecoins as well. Which you can trade for more Ghs, making more BTC, or at least keeping your earn stable as diff goes up. So that should (partially) address any concern about diminishing returns, too.
I was going to post this on my blog later, but since we're ontopic, have a graph of my last month of cex hosted Ghs and what I did to raise it up.
EDIT: the account from which I post this graph also had a variable 3-5Ghs of hardware miners running on it!