I bought one years ago but rarely used it. I mean the information it gave me never made me change the way I ran.
If I was running a half-Marathon I would go at my best speed for that distance and my heart rate wouldn't factor into that.
I've done a 9K ( I don't know why Nike starting making 9K runs, but they did) and I ran my best for that distance also.
That's me though. If it's not breaking your bank to get it then sure.