Another FR305 user here. I don't have the cadence sensor as I have another bike computer on my bike which does cadence. After a while of observing your cadence, you get a feel for it anyway, so it's rare I look now but it was invaluable when I was changing my pedalling style in order to save the old knees.
I have used it for navigation, putting in turn directions from Bikehike. It's not great though, easy to miss it's beep and like others I use Sportstracks which is superb.
It was mainly bought for running as I never knew how far I'd run unless I actually went out on the bike and measured it but it does just fine for the bike too. I have a mount similiar to this. The only problem is it puts a fair amount of strain on the strap when you put it on and tighten it down. A previous HRM monitor broke the strap as I tightened it once, so I use caution now.