I'd say the true answer goes like this:
1. Ponzi scheme (for those who don't know) - payouts are paid from new members investments, the money never invested anywhere and since no real investment can provide X% daily, most of the autosurfs are ponzis, but I said "most", not all of them. Those who are - scams, according to the above defenition.
2. Think of someone running a PONZI scheme, he want to get maximum investments, so he need to pay the first members and then (usually 3-4 month, but it depends on the "X") he have to close the "scheme" when he doesn't earn anymore.
3. According to the above, if you see a new program, which looks good ( no premade scripts, customer support, whois protection, good servers, and so on) you may invest with a very little degree of risk, knowing the program will be run at least for few more months.
4. Legal? - how can a ponzi scheme be legal? Of course not (in most of the developed countries at least, I wouldn't bet on the Pitcairn Islands LOL)
5. There are some exceptions:
TGT is backed by the PIN network and thefore it's not a ponzi,
dadndavesautosurf is not a ponzi,
Now as for HYIPs:
Feeder Fund - 100% NOT a ponzi scam, but a serious long term pool fund
As for the rest, I'm not sure.
6. Running a ponzi scheme is just too easy now, people are ready to give their last money when someone offers them 10% daily, a week after program closes and the admin earned 10-20 thousands. Wanna try? I could have done it easily by myself too, but my conscience doesn't allow me

So I'm here to warn you instead.
7. And finaly what should you do to earn? I can tell you just to go to my site and read there, but I'll do a little more:
If you are planning to invest now - invest in 12daily immediately. I'm pretty sure they'll last for at least a month from now.
If you are seeking a long term invetment with a big money involved (at least few thousands) Invest in Feeder Fund and TGT
that's all for now, but I guess I've already told you too much