Lots of discussion in the Green Room if you search.
The savings are real, and it appears most folks are happy with the water volume. Lots of discussion of some bad internet reviews here:
https://www.hearth.com/talk/threads/geospring-bad-reviews.126616/#post-1709054
but no members came forward with problems. I think it is a small fraction of folks that got lemons, out of a large number of customers that don't post reviews. It is also clear (IMO) there were a bunch of lemon units when the first geosprings came out a few years ago.
The units do need some air >45-50°F to work, so location in a large conditioned or semi-conditioned space is important. They also make some noise, so you wouldn't want them in a living space with a louver door.
I have an AOSmith 80gal HPWH, in my attached garage, that I like a lot after 2 years.
I, like most users, cannot easily see the usage of the unit on my electric bill. If you are running dehumidifiers in the space now (like a basement) the HPWH might offset much of that and your HW is nearly 'free'. I think I am $15-20/mo with 2 teenage girls in the house. In most cases, they use less kWh on an annual basis that a typical solar DHW system costing 3-5x as much (because solar typically uses resistance backup for ~50% of needs).
These are becoming a new standard technology (soon to be required by law when electric resistance HWH >=80 gallons are banned by the EPA). Expect to use 40% as much energy as a resistance tank. Almost like the difference between an incandescent bulb and a CFL.
With oil, you need to know that you can shut down your boiler over the summer without it leaking. Getting rid of my boiler was the best thing I ever did for my house. The darn thing was more than doubling my AC bill in the summer. With the cheap elec in PA, I would use the HPWH year round.
