"It's a Scam"
I'm not sure if this point is being directed at Bitcoin specifically, or just at cryptocurrencies in general in this thread and in the article that poVoq linked. I'll address it briefly anyway. (The previous two sections are the more important ones.)
Many Bitcoin skeptics have described it as a Ponzi scheme or pyramid scheme. If you read about what these schemes actually are, and read about what Bitcoin is, it should be obvious that it's not a match. Bitcoin isn't centrally controlled, so there can't be anyone filling the role of Ponzi or of the directors, at least not for the entire system. It's certainly possible to run such a scheme using bitcoins, but that's true of any currency.
When pressed, these skeptics will usually say something about how everyone invested in Bitcoin is just looking for some group of "greater fools" to pump the price up so that they can make their exit. Like, sure, maybe some people are like that, but to claim that it's some coordinated scam across all of Bitcoin just sounds like a ridiculous conspiracy theory. There are plenty of people that have had ample opportunity to exit profitably and just... haven't?
Being more charitable, these folks largely see Bitcoin as having no future ("it's a bubble"). If everyone is eventually going to get tired of Bitcoin and give up on it, that does make it a really bad investment. That doesn't make it a scam, though.
Anyway that's enough words, I need to sleep. -_-
I may or may not respond if people want to argue over this. Not something I have a ton of patience for these days. If you're genuinely curious and want to pick my brains though, feel free.
Added your instance to the list of specialized instances