I retrofitted one on a house we bought a couple of years ago. I'm really happy with it, although it does add a constant background 'hum'. It wouldn't have been possible however if we weren't doing major renovation -- we tore out & replaced the downstairs ceiling and completely redid the second floor bathroom; that meant we could run the ducts down the corner of the bathroom and then through the joists in the first floor fairly easily. And the attic is a bit cluttered with the ducts.
You definitely notice a difference when you turn it off.
This report [1] suggests that HRVs only actually have a net energy savings in the US in climate zones 4-8 (and such a small amount in zone 4 that it would take several decades to break even).
Is there a good reason to get them if one is in a place where they won't save energy?
I've lived for 8yr now in a home with a Zehnder HRV. It has mostly worked well, though recent wildfires were too much for its MERV-13 filters, forcing me to run it at the very slowest speed and use air purifiers.
You definitely notice a difference when you turn it off.