PBMining offers 5 year mining contracts that are priced waay higher than the contract could earn in the given time frame. Though they seem to be up and running for months now, their "hashing power" have no proofs or whatsoever (pictures of the hardwares and data centres please). If you want to get some nifty profit and at the same time experience some hashing, try trading and using cex.io (I do not promote their service xD).
They have proof as they have their own pool and they prove their hashrate via their found blocks as they would not be able to find as many blocks as they do without as much hashrate as they claim.
What I do agree with is the fact that their prices are much higher then what the GHs that they sell could ever produce. In order to possibly ROI with their contracts the rate of increases of difficulty would need to slow a lot, of which it is not showing any signs of doing.
So instead of being a ponzi they are just scamming their customers by selling their product at overpriced rates. I have noticed that most cloud mining services tend to "estimate" the difficulty rises to be much lower then what would be realistic. Their customers run the numbers with this low increases and buy thinking it is a good deal when if they had done their homework they would see that their assumptions are not going to happen.
