My son received this in his stocking this year and while initially nonplussed, has been delighted with it. He also had 2 other Nanos to join the single one included in the set, and I strongly recommend this as they are even better in gangs!
The Nanos razz around the (small) habitat at speed, and seem fairly robust. Their 12 colored legs are decorative only so no real concerns about snagging and breaking them. the habitat is quite limited, but has inspired my son (7.5yrs) to construct a toilet roll creation for them which works well.
A gimmick? Absolutely, but great fun, no trouble or mess, and doesn't take up a load of space. Makes a superb gift for parties, and is far more portable and appealing to an older age group than the multitudes of rodent interactive pets!
I'm sure we'll end up expanding the habitat, but son is first experimenting with Lego!