Scale is awesome. I got this one:
http://www.amazon.com/Withings-WS-5...32437850&sr=8-1-spell&keywords=Wothings+scale
But there is a cheaper one too:
http://www.amazon.com/Withings-Wire...32437850&sr=8-2-spell&keywords=Wothings+scale
(The only big difference is that the cheaper one doesn't measure body fat)
As for apps: the scale is actually more awesome than that. It doesn't actually communicate directly an app at all. It is WIFI... it sends the measurements to withings.com (and it automatically recognizes WHO in your household is standing on the scale and adds the measurement to their profile).
So you don't have to have any App open at all... your phone doesn't even have to be around at all. Just hop on the scale and done. Perfect.
The Withings App is good though. It gets new measurements pushed to it as they come in... and it adds them into the Health app automatically. The Withings App has a good Apple Watch wxtension too: simply reports your progress for the day.
Everything about Withings is pretty nice. They even already pledged full Homekit support for some of their other products...