There’s a pre-release review on the DC Rainmaker site, and he says he’s currently testing the retail unit so I guess an updated review will be out soon.
I really don’t see the appeal of this over the Garmin though. Less features, way bigger (too big for MTB I’d have thought), no Strava integration. Plus it’s their first bike computer of this kind whereas obviously Garmin are old hands. Plus for anyone who already has Garmin stuff it looks like the Polar will be incompatible with all their existing mounts/sensors.
On the other hand the 520 looks like it’ll be excellent! (As long as the software is solid)