Does anyone have any experience with this?
Can I expect a decent ROI, or likely worthless?
It's as legit as much you pay for it
These cheap lists are scrubbed by bots/scrappers, with no SOI/DOI permissions ... pure spam lists. Some of them are just randomly generated with various tools. Using these can equal to legal problems and getting into trouble with your email service providers.
Of course people are doing this anyways, using some bullet proof servers located in Russia and sending millions of emails to these addresses. Large part of the addresses is dead, the bounce rates are huge, open rates are minimal... but since they mail in millions, they can still make $$$ from it.
Whether you wanna go that route is up to you, I certainly wouldn't personally.
IMO you are asking too generic question to expect some decent answer.
ROI depends of the database quality, offer, your ESP setup etc.
My assumption is that these emails are already publicly available and probably spammed to death by gazillion other marketers selling "original" RayBan sunglasses, Yeezy sneakers, dick pills and other products you can find in your Spam folder. 
I don’t prefer it
In my opinion, you would be buying the email list have 500-1000 emails maybe over for (5-50$). but when sending email not have clicks or have problem with email provider like spamming
IF you have product you need promotion with email traffic i prefer using SOLO Ads
test ,test and test 
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The offer is 1M adult leads for $100.
The description just says he's got a lot of experience running email campaigns, and this hints that they were generated through affiliate opt-in. Maybe.
Also the sellers from a western geo, which adds something.
(What I mean is, you can be fairly sure that the ROI from some seller jurisdictions will be 0)
It will be rubbish, 1mil for 100 bucks. But yeah some people are very creative with rubbish.
What would be the going rate to sponsor/rent a good 1M list?
And what kind of CTR could I expect from each ($100 and $xxxx)?
If the numbers add up, it makes little difference?
So I probably shouldn't expect too much from the 4.2bn email blast that cost $50?
The only possible outcome is that whatever you promote that way is about to get flagged buy all the anti-spam tools out there