I have a ZNA. I was one of the lucky ones that snagged it within the 3 minutes of selling out. I love it. It's not my first Zen device and the care in manufacturing shows in this one as well.
The button does not have any problems "sticking." There may not be a spring but the button itself is "sprung", though I don't think there is a spring on the board, but that button itself will not easily be activated if it is not being pressed. If you are putting it in your pocket or purse you should be clicking 5 times to lock it.
A nice feature that this has that the dna 20 doesnt is the power lock feature. Given the way you hold the ZNA this is practically a required feature. Without it you would have to gingerly use the device so that your palm/fingers dont change the power setting.
.../snip
Question? Any reason this wouldn't be a good mod for carto/tank users?