Nano Challenge is an attractive looking gadget that I bought from Amazon after seeing it in a gift catalogue. I've been playing it for a few days now - it is very playable and my hope is that my memory is being 'excercised' whilst playing! It looks attractive, the buttons have a pleasant positive pressing action and the ability to switch the sound on or off is useful, as I can imagine it could get annoying for anyone else in the room. The sound effects are proper synthesised sounds and whistles, not just plain 'beeps'.
My only niggle is that if you play it in the pitch dark, the coloured buttons do not actually glow the colour they appear in normal light. The blue, dark green and light green buttons all glow the exact same green colour and both the red and purple buttons glow the same red colour. This means that if you're trying to remember the sequence in your mind by mentally 'saying' the colours in order, or using the colours to help you play, it's impossible to do so in the dark!
It comes with a keyring, but I removed this with some pliers - whilst the game itself is not massive (6.cm long, 4.5cm wide and 2cm deep) in my opinion the unit would be too large to feasibly add to a bunch of keys.
An excellent stocking filler, and a great alternative to more expensive games such as the Nintendo DS if you're going away or are worried about losing/damaging them.