I don't see Cex.io as a profit center in it's own right but rather I use it as a place to store some of my BTC
if I have my BTC sitting in a wallet, it earns nothing
if I use some of my BTC to buy GHS then it makes at least some BTC
when and if I need the BTC, I just sell the GHS and withdraw the BTC back to my wallet
I don't have all of my assets in this primarily because I don't want to trust any entity too much lest I get Gox'd
so far, it has worked out positively for me...your mileage may vary.
Oh God no, don't do that. Look at the graph for the price over the last 6 months. It will just keep going down faster than anything you might be mining. You're basically giving money away to the flippers. If you're going to get into CEX you HAVE to flip to make a profit. Otherwise just put your money in cold storage.
By storing BTC in cex.io and use them for mining, we get these 2 values:
X = initial ghs buy value
Y = final ghs sell value + mining profit - maintenance fees - risk of cex.io/ghash.io hack or shutdown
if you think Y will be greater than X, go invest on cex. If not, keep them on offline wallet.
The only certain thing is that noone can say which it will be.