No one knows a good Ponzi until the money is gone.
As i see it, you are making 2 bets with your money at pbmining.
The first bet is that pbmining is not a ponzi and the second bet is that the bitcoin price will rise.
First bet is out there, but second one is not, as you pay in BTC and get BTC back form them. No fiat involved in any way, so price of BTC does not matter.
Yes, you can say "But if BTC value rises, and i ROI only 90% of BTC back, i earned some money". Well no, you lost as you would have more if you didint spend it on PB.
There is second "bet" that every PB user takes tho....diff. If diff jumps in crazy numbers, it will eat what you invested and you will loose no matter if they are ponzi or not.