FlOw is a game about piloting an aquatic organism through a surreal biosphere where players consume other organisms, evolve, and advance their organisms to the abyss.
As part of Jenova Chen's thesis research, flOw inherits an embedded design of active DDA (dynamic difficulty adjustment), players with differing skill levels can intuitively customize their experiences in the zone and enjoy the game at their own pace. 'Addicting' is the most common word its fans use to describe it.
More info here: http://www.jenovachen.com/flowingames/fl owing.htm