I recently started doing the stepper and noticed the improvement compared to occasional physical activity that I inconsistently do from time to time (e.g. sports outside with the kids).

Edit: typos

I've seen them in various organic grocery stores.

I was gifted a WHOOP 6-month subscription in December 2020, and I used it from then until the end of that 6-month term. This was the 3.0 platform.

It was fine for the price - $0 - and provided me with HRV and a "sleep score", which is of interest to me and other athletes as one indicator of "fitness."

I did not - and will not - pay $360/year for WHOOP. I could actually buy a pretty nice state-of-the-art new real fitness tracker for less than that, and the features that were somewhat novel to WHOOP are now standard on other hardware platforms (Garmin, Polar, others). Note that with many Fitbits and clones, you get GPS built-in so you can leave your phone at home and do your thing.

It differed from other fitness trackers mostly by what it didn't have, and DC Rainmaker has exhaustive reviews up to and including the current 4.0 release.

In short, it's not a fitness tracker like a Fitbit Charge 4 or a Garmin Venu 2 Plus - I own both - because it doesn't track steps, has no GPS (you need to lug around your phone for that), and relies entirely on HR for what it tells you - all for a pretty heavy price tag. It also doesn't pair with 3rd party HR hardware (such as the usually more accurate EKG-based ones like a Garmin or TICKR chest strap) and relies on wrist optical sensors.

There are a number of metrics WHOOP reports, but they are simply rehashed versions of metrics developed by Andy Coggan and others some time ago.

TL;DR: WHOOP is overpriced and not feature-rich as a fitness/activity tracker. If you get one for free, try it out. You can easily get the same functionality and more for half the price of a yearly subscription.