Plainly awesome. I love the interactivity and the tactile feel of play. I have all kinds of gadgets for work and personal use, but the Sifteo cubes are in a class of their own: I couldn't put them down the whole day I got 'em. I bought two sets: one for me, and one for my daughter. I think they make a great gift for an adult (I'm 35), or for a child. My daughter is fascinated by the concept of little cubes that know how to "touch each other" or how they "talk to each other." Doesn't matter how she describes it, she's excited about an educational game (she loves her Sifteo Scrabble) and she's excited about something scientific. Mission accomplished!
Despite my clear enthusiasm about the product (no, I don't work for Sifteo), there is one thing I wish were done differently. The cubes require a live connection to a PC to work, which sometimes is limiting (what if we're not around to setup my daughter's cubes?) But no matter, I would still buy them just the same, given my kid's positive response!