PowWeb sells 1 GB of bandwidth at a rate of $0.03. That price that you pay is supposed to cover not only traffic bill, but hardware, customer and technical support, among other 100 things. PowWeb, you can say, is overselling their services. However, the impression is that the majority of their clients are satisfied with service. Keep in mind, however, that those clients do not even come close at consuming those 300 GB, due to number of limitations they establish for each account. Due to this, PowWeb is able to attract a lot of thousands of consumers through great offering, but the reality of the matter is - if all those users started to consume all 300GB every month, PowWeb business model would quickly become unsustainable.
Please note that I have never had any direct experience with a company and only express my personal opinion.
Best,