It replaces the Basis B1 watch and Jawbone UP that I have been wearing together, one on each wrist. I got the Basis in October 2013, and the Jawbone UP in November 2014. Replacing them because they died after being soaked in rinse water with my wetsuit. :-(
I considered getting a new Basis watch and a new Jawbone UP, since the combination worked well, except for the large UP being a bit tight.
But I decided to switch to the Withings Activite Pop, mainly because it was one of the few activity monitors that can automatically track swimming as well as walking.
This blog to track experience.