Allright, I've had it a few days now so I can give you a small report.
+For normal fitness training, it's pretty cool. By measuring interval between heartbeats it can give you estimates on your breathing rate and oxygen consumption, which I've never seen in another HRM. You just tell it your vital capacity and it tells you how much you use oxygen in training. Propably not 100% accurate, but still a good reference
+It looks reasonably cool (by that I mean not hideus like most polar computers, but it's not exactly pretty). The menus and functions are clear, easy to read blah blah...
+supposedly you can get the HR data transferred wirelessly in real time, so writing own software for different functions should be possible.
-Ok, now here's the thing. The watch is water resistant to -100m. Cool, now what they don't advertise, is that the sensor belt is only to -20m! More specifically, it is mentioned in the manual that "you can swim with it, but not dive". Bah..What's the point of having a -100m water resistant watch if you can't dive with it
Well, I guess for the mundanes it would never even occur to take it down to such depths.
-Does not work in chlorinated water (I don't know if it is even technically possible). I was able to get a reading by putting it under my wetsuit and keeping the suit relatively dry. Even then I had to bring the watch right next to the belt to get a signal.
-My guess would be that it would not function any better in sea water. Maybe there's small hope for freshwater.
Anyway, great tool for training in land, but offers really nothing for freediving training. That's truly dissapointing. I think I will call them up and see if I can talk to someone technical and ask if there's ever any hope of getting a HRM that works underwater. Signal wise it should not be a problem, they have wireless transmitters in the D9 for example. But isolation of the electrodes in salty water is probably a bigger one...
Here's a graph of an empty lung dive I managed to log...