I tried only one site:
www.CrazyPonzi.com/?r=228. It is like open-source Ponzi scheme, with 120 hour repay window. If your bet gets paid by newer 'investments' within 120 hours, you get 126% (nominally 140% minus 10% of bank commission (of which 50% goes to referrer of winning account, if any)). You can watch your investments expiring or getting paid on one big dashboard.
You can withdraw your entire account balance by ticking a checkbox and waiting for daily withdrawal. They do exactly what they promise and gladly accept suggestions from players. Creators of the game constantly work towards greater transparency to build trust. It seems they realize it is better to build trustworthy gambling site.
Speaking of gambling: some of players will inevitably loose. I had the luck to win - I 'invested' 0.005 XBT and withdrew over 0.008 XBT, gaining ~71% in two weeks. This is fairly OK for me :-)
You risk nothing. Or - almost nothing, bets are as low as 0.001 XBT, so please do not hesitate! Use my referral link
www.CrazyPonzi.com/?r=228, create account and play! (You can PM me for some winning-strategy tips, of course) :-)
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Cheers,
Szycha.