Relaxing, beautiful, simple.
I picked up Flow as part of PSN's half-price sale on downloadable games. Four dollars is totally reasonable for something as simple and fun like Flow.
A friend and I played through the game in a few hours. I think we were both surprised by how eager we were to see how the creatures grow and to see what other animal forms you unlock as you progress. It's not really "addicting" per se, but it definitely has that "just one more turn" compulsion of grander games like Civilization or World of Warcraft.
It doesn't hurt that the game is simply stunning to look at, even moreso on a HDTV. We had friends over for New Year's Eve, and everyone was having a blast watching, as well as playing.
The regular $8 price tag seems a bit steep for something as modest as Flow, but if PSN ever has another half-price sale, Flow is definitely worth checking out as a refreshing game experience and one of the best applications of Sixaxis motion I've seen yet.