If I send 0.1BTC and then another person sends 0.001BTC will I get 0.12BTC ?
If you send 0.100, that creates a pool of 0.1000 - and you're at the top, because no one else is in the pool yet, so yes, when others put their money in, you technically will get a payout of 0.12
BTC - at least until the site gets 'hacked' or 'loses the
BTC wallet' or whatever.
It's not really a Ponzi scheme. A 'true' Ponzi scheme is where people are tricked into believing their money is being invested, and that they are being paid a return. In fact, the new people are paying off the early investors - until it eventually crashes. But hey, it makes for good press, haha.
It's basically like a gambling game, except you know if you get in early, you'll win, while the later in the game you start, the more likely you are to lose out. In gambling, unless you get lucky, you're always going to lose, because the odds are stacked against you.
Poker, by the way, is not considered gambling (it's just like a Ponzi scheme - all of the people who don't know how to play end up losing their money):
http://www.henrybuell.com/gaming-legislation-makes-poker-history/