Define 'serious'.

They are all ponzi-based scams. What would possibly make them legitimate? Nothing.

If you don't know this then you're too gullible to be using the internet.

If you do know this then you are guilty of being an accomplice in fraud and theft if you manage to make more than you 'invest', because those extra satoshis you receive will have been taken from other users who will lose their funds when the scheme collapses, which it will do because of math.