I've just bought Z710 to my wife...Previous she had an old Z600, she love her Z710 more and more, I was planning on changing the firmware to W710. But I believe, all what you gain is the heartbeat meter..
Shes not a workout person, so I guess I skip it..
Anyway, when I looked at pictures and so on in tests, until I held it in my hand in the shop, I thought it was much bigger in size..Dont know why..

One disadvantage though..Battery capasity...Bad!!..
Needs recharging every second day..