The Otamatone is a fun instrument to have laying around in your house. For its price there is nothing major to complain about. It has a good range and I use it to train my ears a little bit when figuring out songs since I've never played an instrument that allows for a free range of micropitches.
The only minor complaint is the responsiveness of the playing. Sometimes I would describe it as buggy. But it is understandable, and if you want to take this seriously, there is the deluxe version which seems to handle the responsiveness a lot better.