I'm intentionally not calling it a Ponzi. People would then cling to the conventional definition of a Ponzi scheme and come up with arguments like "but you can get rewarded without investing anything, so it's not a Ponzi". However, we'll get to that argument as well.
Current distribution of wealth
Out of over 47000 accounts, 46 accounts hold 64.51% of Steem Power. That's a very unequal distribution of wealth, but doesn't necessarily make it a scam in itself. Let's see what does.
Continuous generation of wealth
For every 1 unit of wealth generated as a reward (author, curation, witness, etc), 9 units of wealth are generated and distributed to Steem Power holders.
So while indeed, users do get rewarded without investing anything, for every dollar you see distributed on the website, 9 dollars are given to the already wealthy behind the scenes.
To help that sink in, let's divide the userbase between the top46 wealthy and the rest of users and see how much each group is rewarded with each generated token.
Of every token, 90% goes to Steem Power holders. The top46 users hold 64.51% of the Steem Power. So their share of each generated token is is 0.9*0.6451 = 0.58059, that is, over 58%.
Even if none of the curation, author, comment, witness, etc rewards were fed back to the top46 wealthy, they will still receive 58% of every generated token. In practice, it's obviously more, as they also receive a high portion of these mentioned rewards in addition to the 58% they get for doing nothing.
The coverup
10% of every generated token is divided between the rest of the users, mostly proportional to the...will of the "Steem rich". That's what you see in the interface as rewards. This is pretty much just a tiny bit of the newly generated tokens, distributed as a bait to lure more users into investing and cover up the true pyramid of Steem.
So while you could reason that 10% of the generated tokens is backed by the value of "community and content authoring" (if you accept that that a makeup tutorial has a market value of thousands of dollars), 90% of the generated wealth is backed by nothing but the money new investors are expected to bring in.
And bringing in new investors they try. Why do you think most author rewards go to influental people lately? People with serious follower bases on various social media platforms. Yes, you guessed it, to make them bring in more investors.
Summary
So, we've seen that very few people - devs and early birds - hold most of the "Steem wealth" (top of the pyramid).
We've estabilished that the top wealthy users receive most of the newly generated wealth without contributing any kind of value. In order to cash out these tokens, they must convince new users to invest in the platform.
The only difference to a pyramid scheme is, 10% of the generated tokens are distributed as a show to lure in new investors.
So my advice: If you want, try and take what you can of this 10% and cash out as soon as possible, but under no circumstances invest anything to enter the pyramid - it will collapse sooner than you think.