On the downside, like much in the HVAC world, it requires new construction or a fairly major retrofit to take advantage of it. My guess is that it's only going to make sense for new higher-end construction where other attention is being paid to energy efficiency. (Though I'd love to see a window box HRV for renters, just like window box air conditioners.)
If you haven't heard of HRV / ERV yet, this video shows how they work in an impressive but simple demo with the 4 temperature sensors in the last few minutes: https://www.youtube.com/watch?v=QOSelUK6dpQ (Also compares aluminum core heat exchanger vs. desiccant enthalpy wheel)
For the DIYers, here's a forum full of people building their own HRVs: https://ecorenovator.org/forum/showthread.php?t=891
Best would to get a blower door test run and see what your ACH50 rating is before considering one.
There were also a few days this summer where it was 50°F and low-humidity outside, but 78°F inside, my preferred temperature was closer to 72°F, and I wished I had some better way than "open a window and stick in a Lasko box fan" to cycle in cool outdoor air.
I've seen new ERV units under $1,000 and some pop up used on Ebay for great prices.
You definitely notice a difference when you turn it off.
Is there a good reason to get them if one is in a place where they won't save energy?
Here's a US climate zone map [2].