Rarity aside, CNET notes that it works as promised. Bring your Apple Watch close to the machine and it'll ask you to confirm your activity, and syncs activity after that. All the data your watch would normally collect shows up on the exercise equipment's display, including activity-specific data like flights climbed on a stair stepper. The info reflects the Apple Watch's heart rate measurements as well as the height and weight you supplied in the Health app, so you don't have to keep a firm grip on handlebars or punch in stats like you do with conventional fitness hardware. And if you forget to sync before you start, don't panic -- you can still pair later into a session, including afterwards.
Support will spread soon enough, with 130 other Life Time gyms (focused initially on gyms opened in 2017) across the US getting GymKit starting in 2018. Equinox gyms are also upgrading in 2018, starting with New York City. However, it could be a very long while before the technology becomes ubiquitous. As any gym rat can tell you, facilities tend to hold on to equipment until the bitter end, waiting until it either breaks or is woefully outdated. Your local gym probably isn't going to upgrade just to make sure its machines and your Apple Watch exist in perfect harmony.