CEX.IO Bitcoin Mining Return on Investment Calculation
Very good summary here:
http://ppalme.wordpress.com/2014/01/20/cex-io-bitcoin-mining-return-on-investment-calculation/

Altogether from my point of view one can say that mining does not have any benefits for the customers.
Here you kann see how the price per GH/s reduces 10.5 in 7 Months:
https://bitcoinwisdom.com/markets/cexio/ghsbtc

The earning through mining (for example with 100GH/s) is about 0.0003 BTC in 5,5h, Maintenance included, 0,2% Fee not.
I will test the Cloud Mining of CEX.IO some days more, but it looks like a useless invest.

I think with this low earnings the Mining-ROI will never fund the loss of the invest for the GH/s.
And Difficulty increases two times a month and then you have to reinvest in GH/s to get the same earnings like before.
Maybe a good ROI is possible with "daymining" with very high investments > 5TH/s.
But the risk compared with the possible benefit ...

Sorry, currently i´m not convinced.