I mined with cex for a couple of weeks and enjoyed trading hashpower and earning bitcoins rather than just having them sit in my wallet.

The problem that I had with them though is that there was no indication of how or when CEX added to the supply of hashpower. I asked them and they said they only do it when the difficulty increased, but basically it was totally opaque. There is no way that I could tell what was the total supply of hashpower in the market or how it was changing.

Despite that, I traded for a while. I thought I basically had the market figured out. With the difficult changing at known intervals I though it would be relatively simple to sell just before the difficulty changed (supposedly the high) and buy just after the change (supposedly the low). But during the last adjustment the market didn't act like that, which gave me the heebie-jeebies so I sold everything (at around 0.072 BTC/GHS) and I'm super glad I did because it crashed hard the last few days.

The motto of the story is: a market is a market. If you plan to trade in it then you're doing it to make money and you need to invest time to learn it and do it properly. Basically I don't have the time, so I'm not willing to take the risk any longer. It was a good learning experience though.