The first Otamatone that got delivered was defect (mine stayed at a low volume all the time even when turning the volume knob. According to some websites this specific Otamatone has a lot of defective ones), but the customer service of Thomann really helped me through this one and I had a new one soon after (this one did work correctly).
I purchased mine just out of curiosity for the product itself. After playing it, I can say it's really fun and it brings a smile on the faces of friends when they see it for the first time. To me it was pretty easy to get used to, because it's very similar to playing a piano/keyboard.
The Otamatone doesn't have many features. Just a volume knob and a button for switching between 3 modes that change it to a higher/lower range of notes.
If your reason to buy this is to have a first instrument , then you probably should avoid this and get a real instrument first (although, with some practice you may still be able to play it, but to me something like a guitar or keyboard would be a better option than a Otamatone).